Recommended Reading
Classical Advaita

There are very many translations and commentaries on these, either singly or in groups. There are not very many versions of the 'Complete' Upanishads, if it could be agreed what this means exactly, since there are certainly more than 100 separate ones. Upanishads such as the Brihadaranyaka (bRRihadAraNyaka) or Chandogya (ChAndogya) can run to as many as 1000 pages, including commentary, while some like the Tejabinda are only one or two pages. Because there are so many, it is difficult to recommend specific ones. The best thing to do is to visit a specialised bookshop and browse. See Buying Books.

If you want to but collected works of the major upanishads, there are a number to choose from:

A version, with no Sanskrit, no literal translation and no commentary is 'The Ten Principal Upanishads' put into English by Shree Purohit Swami and W. B. Yeats. This can definitely be recommended but should be read as poetry rather than as an aid to finding out about Advaita.

The best value for money I have discovered is a little book 'Four Upanishads' by Swami Paramananda. It covers four of the principal Upanishads - Isa, Katha, Kena and Mundaka. There is no Sanskrit and not all verses are commented but the commentary that is provided goes straight to the heart of the matter.

The definitive version of eight of the major Unpanishads is probably that translated by Swami Gambhirananda and with the commentary by Shankara. It comes in two volumes and is published by Advaita Ashrama. I've always been put off this version because it appears too academic. The publisher's comment reads:
'Study of these commentaries by such an ingenious philosopher-saint like Shankara is indispensable for the proper evaluation of the Vedanta philosophy. With the text of the Upanishads in Devanagari, a lucid and faithful translation of the text and commentary, relevant notes, reference to quotations, and index to texts, this volume proves to be a vade mecum for all students of Vedanta.' (Recommendation by Michael Reidy.)

If you want to look at individual upanishads, the major ones are the Kena, Katha, Isa or Isavasya, Mundaka, Mandukya, Prasna, Taitiriya, Aitareya, Chandogya and Brihadaranyaka. Of these, I would recommend the first 3 to begin with. The last 2 are very long. The Mandukya is possibly the most important, with its commentary (karika) by Gaudapada, but it is quite difficult so should not be attempted straight away.

There is a particularly good version of the Kenopanishad (Kena) with commentary by Swami Muni Narayana Prasad.

Swami Chinmayananda has commentaries on most of them, including this one on the Kathopanishad (Katha) published by Chinmaya Publications.

The Mandukya is essential if you want to learn about states of consciousness or the ajati theory of creation.This one has Shankara's commentary (on Gaudapada's commentary), and is translated and commented by Swami Nikhilananda (available from Advaita Ashrama). This really is only for the serious student, however.

A much more approachable version is the one commented by Swami Chinmayananda.

The book 'Dispelling Illusion' by Douglas Fox is a more general look at Gaudapada and his ideas as presented in the karika of the Mandukya.

Finally, this new version of the Mandukyakarika by Raphael is recently available in English. I must admit I found it hard going at times but it is ultimately worthwhile and I expect to re-read it in the future. Overall, I was certainly convinced that he knows what he is talking about, though I did feel that somethimes he does not try hard enough to convey that understanding with sufficient clarity to us ignorant ones. Anyway, given that all the commentaries that I have so far seen are difficult to understand, this is one of the better ones and worthy of note.

Rather than reading the Upanishads themselves, you might prefer to read a modern 're-telling' and interpretation. The following are extremely good and are highly recommended. The author, Ananda Wood, is a disciple of Atmananda Krishna Menon, and he provides exceptionally lucid descriptions of difficult concepts.

From The Upanishads - Free translation of selected passages from a number of the Upanishads into blank verse, along with some occasional prose. Divided up according to clear topic headings. An original adaptation to make them more accessible to the modern reader.
Free Download in PDF format.

Interpreting The Upanishads - This focuses on particular ideas from the Upanishads, and explains how these ideas can be interpreted. For each idea, selected passages are translated and placed for comparison beside much freer retellings that have been taken from the first book. The Sanskrit is often referenced with explanation of alternative translations.
Free Download in PDF format.

Both books were available from Zen Publications in India (Pune and Mumbai) but it is likely that they have now sold out. There is also no on-line ordering facility here. They were also offered at Nataraj Books in Springfield, Virginia, from where it is possible that they may be ordered on-line. They are not available from Amazon.com though they are still advertised at Amazon UK.

There are many different translations and commentaries on this classic work, where 'many' = tens if not hundreds. Some merely translate the Sanskrit, with varying degrees of accuracy and artistic licence. Others provide several pages of commentary on each verse. I have only 7 different versions so it is perfectly possible that many of those I have not seen are excellent.

The most authoritative version is probably the one that includes all of Shankara's commentaries - translated by Alladi Mahadeva Sastry. Unfortunately, though undoubtedly authoritative, it is not the most readable and I would not recommend it to other than the most serious students.

Swami Dayananda has written 'The teaching of the Bhagavad Gita'. This is really using the Gita to present an overview of Advaita and it uses verses as illustrations rather than covering the entire book, verse by verse. It is, nevertheless, an excellent book and I can thoroughly recommend it. He also provides a study course on the Gita, using extensive notes, which are excellent (available from this site).

Winthrop Sargeant has a version without Commentary, which has both original and Romanised Sanskrit, together with the meaning and grammar for each word. A number of pages may be read at Amazon.com.

Swami Chinmayananda has a voluminous edition published by Chinmaya Publications. Unfortunately, I have not yet read this so cannot make any positive statements. It looks likely to be very good, albeit possibly a little verbose. It does suffer from a slight drawback in not having the original Sanskrit presentation of each verse. An excellent html-based package, containing the complete text is available for download. This does contain the Sanskrit as well as additional study notes.

Overall, I would recommend the version with commentary by Swami Chidbhavananda. Each verse is given in Devanagari, followed by Romanised Sanskrit and then a word for word translation. A full commentary is then given, often using excellent metaphors. Frequently, relevant quotations from Sri Ramakrishna are then presented. This book is available from Blue Dove.

Brahmasutra

This is the third branch of the prasthana-traya - you will need a specialist bookstore to locate it. Not that, since you will presumably only be interested in the Advaitic interpretation, you will want the one with the bhashya by Shankara.

There are a number of versions and I am only acquainted with the one with commentary by Swami Gambhirananda (available from Advaita Ashrama). It is an exceedingly difficult book to read and, though it contains some of the most profound philosophical analysis, it is certainly not for the beginner.

You can also join the Advaitin E-Group and follow the exposition by K. Sadananda. He is posting periodic commentaries on each of the sutras, based on lectures given by Swami Paramarthananda of Madras, together with his extensive exposure to and understanding of the philosophy. These are likely to continue for some time and archives of all of the earlier material may be downloaded.

Other classics of Advaita

Astavakra Gita (aShTAvakra gItA or aShTAvakra saMhitA) Certainly not shruti and not really smriti either but I would put it into the same class even if traditionalists might frown. It is not known when, or by whom, the original work was written. It is named after the mythical Sage who appears in the Mahabarata and the Vishnu Purana, both very old scriptures. It is thought, however, that it was probably written more recently, either around the 8th Century or even as late as the 14th, by a follower of Shankara.

I would recommend the version translated and commented by Swami Nityaswarupananda. A relatively small, thin and cheap version, easily fitting into the pocket, this can be carried around and is a source of the most wonderful uncompromising statements on pure Advaita. Complete with Devanagari Sanskrit and word for word translations, this is my favourite book in this section.

Again, Swami Chinmayananda has a commentary. A much weightier tome, this version may be easier to find.

Ramesh Balsekar also has a commentary on this, called Duet Of One, which I have not yet read.

The modern translation by Dr. Thomas Byrom is available on-line with its simple yet luminous prose - highly recommended. It may also be downloaded as a pdf file from the Satsang with Ganga site, where there is also a more literal version, also as a pdf file (note that the two liks are the wrong way round at present).

There is also a translation by John Richards available on-line at Realization.org.

Atmabodha (Knowledge of Self) - (attributed to) Shankara.

Highly recommended, though only available from the Author's organisation 'Vedanta Life Institute', Sri Parthasarathy provides original Sanskrit with word for word translation. The book is also liberally sprinkled with excellent metaphors and stories. The nature of the Real Self is dealt with at length.

Another version of Atmabodha, 'Self Knowledge' is that translated by Swami Nikhilananda. It has copious notes and a very long introduction on Hindu Cosmology, Asramas and Realisation. Very Useful. This recommendation courtesy of Michael Reidy. 10 sample pages to read at this link.

Vivekachudamani (Crest-jewel of discrimination) - (attributed to) Shankara. Shankara is the nominal author of a number of books that are considered classics, almost of value comparable to those in the above section. This is probably the most famous.

A simple but useful book that is not exactly a commentary, since no actual verses are presented, is that by Swami Prabhavananda and Christopher Isherwood.

Upadesha Sahasri (upadeshasAhasrI) (A thousand teachings) - Shankara. This is one of the few books that all scholars seem to agree was definitely written by Shankara. It requires some effort on the part of the reader but covers the subject of knowledge of the Self with thoroughness and obvious authority.

The version from Sri Ramakrishna Math, translated by Swami Jagadananda, presents the Devanagari and English translation. One very useful extra is that footnotes are provided listing the Upanishads from which each of the very many references in the text derives.

A somewhat more obscure book, also attributed to Shankara, is worth looking out for. It is very small, easily carried around in one's pocket, yet merits re-reading and study. It's title cannot even be written satisfactorily in the Roman alphabet, so here it is in ITRANS: 'dRRigdRRishyaviveka', translated as "an inquiry into the nature of the 'seer' and the 'seen'". It addresses the topics of the illusory self, the universe, maya and samadhi.

The version translated and annotated by Swami Nikhilananda and published by Sri Ramakrishna Ashrama is the version with which I am familiar and can highly recommend. It is available from the Vedanta Society of Northern California for a mere $1.75.

Another Indian classic, often referred to by Ramana Maharshi, is 'Tripura Rahasya: The Secret of the Supreme Goddess', translated by Sri Ramananda Saraswathi. This has been compared to Plato's 'Republic' since, like the Greek classic, it outlines the ideal city-state of a characteristically Indian utopia. A customer review at Amazon states:
"Through a series of stories that can be read with amusement and the greatest of imaginative pleasures, the tales also provide a series of wisdoms and insights that illustrate and represent theological implications within Hindu theology and Goddess traditions." - Recommendation by Michael Reidy.

One other general book on Advaita recommended by Michael is 'Methods of Knowledge According to Advaita Vedanta' by Swami Satprakashananda. Described as 'an excellent study and reference work presenting the means of valid knowledge accepted by the Vedanta tradition which lead to Self-realization', this book may be difficult to find. It is available from the Vedanta Society of Northern California, if you cannot find it elsewhere.

The 'Council for Research in Values and Philosophy' publishes a variety of books in a number of areas quite unrelated to Advaita. One book however (IIIB.4) covers the work of Shankara in considerable depth. It is written in a scholarly manner with detailed references but is nevertheless very readable. It is 'Self Realisation (Brahmaanubhava): The Advaitic Perspective of Shankara: Indian Philosophical Studies IV' by Vensus A. George. It may be purchased but is also available for free download for personal study.