To work on the Upanishads, one might well begin with one or two of the translations of the Upanishads which come with general introductions and often follow one or more of the commentaries. See, for example, the new translations listed below of Patrick Olivelle, the classic translation of R.E. Hume and that of S. Radhakrishnan.
Allchin, F. Raymond. The archaeology of early historic South Asia: the emergence ofcities and states. New York: Cambridge Univ. Press 1991
Between this book and the one cited below by Erdosy, one should have a good grasp on the period of the Second Urbanization. Scholars use this name to refer to the period in the first millenium b.c.e. when the plains of North India saw the growth of cities and large socio-political formations ("states"). Some, including Olivelle, consider the processes of urbanization and the dramatically changing sociology of North India during the period to be an important, if understudied, aspect of the Upanisads.
Brereton, J. "Tat Tvam Asi in Context." Zeitschrift der Deutschen MorgenlandischenGesellschaft 136, 1982: 98-109.
tat tvam asi is the famous mahavakya (great aphorism) that has been traditionally translated and interpreted as "That thou art". Brereton discusses the problems with this interpretation and offers a more grammatically consistent translation.
Dandekar, R.N. Vedic Bibliography. 5 vols. 1946--, Bombay and Poona.
This is a "Vedic" bibliography in the fullest sense of the term, including within its purview the early Upanisads. As the years progress the volumes get larger and more indispensable. Especially useful for looking up articles on specific Upanisadic subjects. Annotated.
Edgerton, F. "Sources of the Philosophy of the Upanisads." Journal of the AmericanOriental Society 36, 1917: 197-204.
"The Upanisads: What Do They Seek, and Why?" Journal of the American OrientalSociety 49, 1929: 97-121.
Accessible and solid treatments of some core issues in Upanisadic studies, particularly the second of the articles cited. The name says it all.
Erdosy, G. Urbanization in Early Historic India. Oxford: BAR 1988.
An excellent source for the socio-cultural context behind the composition of the Upanisads, which is also important for a study of the rise of Buddhism and Jainism.
Hume, R.E. The Thirteen Principal Upanisads. Oxford: 1921.
Now eclipsed by Olivelles translation but still useful, especially insofar as it provides a glimpse at what most English-speaking readers had access to before Olivelles work.
Kuiper, F. B. J. Ancient Indian Cosmogony: Essays selected and introduced by JohnIrwin. New Delhi: Vikas Publishing House 1983.
This collection of essays has one in particular that is valuable for a study of the Upanisads, "The Ancient Aryan Verbal Contest" (first published in Indo-Iranian Journal vol.4, no. 4, 1960). This essay traces out the history of the "brahmodya" or contest-dialogue from the earliest Vedic texts up to the late Vedic period. While not about the Upanisads, per se, it cant be beat for the clear way it lays out the tradition of the verbal contest and its ritual, metaphysical implications. Also of interest in this collection is "The Basic Concept of Vedic Religion", a very concise and clear outline of basic Vedic religious ideas first published in History of Religion, University of Chicago, vol. 15, 1975.
Oldenberg, Hermann. The Doctrine of the Upanisads and the early Buddhism. trans. into English by Shridhar B. Shastri. Delhi: Motilal Banarsidas, 1991.
An influential and erudite late nineteenth century look at Upanisadic philosophy and how it bears upon the rise of Buddhism. Dated, of course, but still remarkably interestingcovers a wide range of topics.
Olivelle, P. The Early Upanisads: annotated text and translation. New York: Oxford University Press,1998.
The state of the art as far as Upanisad translations go. This invaluable work is highly accessibly and absolutely indispensable for both the beginner and the expert. The important thing about this edition for the non-Sanskrit reader is the inclusion of detailed annotations. In these annotations one can find references to any pertinent articles, as well as Olivelles own well-reasoned take on any particular issue. Incredible index, for example ("termite: Brhadaranyaka Upanisad 1.4.7; Kausitaki Upanisad 4.20"!)
Olivelle, P. "Unfaithful transmitters: philological criticism and critical editions of theUpanisads" Journal of Indian Philosophy (Netherlands) 26, 2 (1998: 173-187).
This is an interesting article indeed. It makes a strong case for the value of listening to the Indian commentarial tradition, which may seem self-evident but to many western scholars, particularly in previous generations, is far from it. He shows the ways in which western editors could have definitely benefited by a respectful reading of the commentaries. This article is thus useful not only as a source for understanding how the Upanisads have been transmitted to us, but also in the way it addresses important methodological issues.
Olivelle, P. "Young Svetaketu: a literary study of an Upanisadic story" Journal of theAmerican Oriental Society 119, 1 (1999: 46-70).
This article takes a story that occurs in both the Chandogya and Brhadaranyaka Upanisads and examines the different ways the story is told. This exercise proves enlightening, particularly with regard to narrative strategies that typify, on the one hand, the Chandogya Upanisads conservative Kuru-Pancala-centered outlook, and, on the other, the Brhadaranyaka Upanisads more challenging, frontier feel.
Renard, P. "Historical Bibliography of Upanisads in Translation." Journal of IndianPhilosophy 23, 1995: 223-246.
Interested in looking at the various ways the Upanisads have been translated over the years? This is a good place to start.
Renou, Louis "Le Passage des Brahmana aux Upanisad" Journal of the American Oriental Society 73 (1953: 138ff.).
Renou, L. and L. Silburn. "Sur la notion de Brahman" Journal Asiatique 7 (1949: 46ff.)
Highly recommended for those who can read French. I do not think either of these has been translated, but I may be wrong. Both of these articles, in typical Renou fashion, extend far beyond the confines of Upanisadic studies. As such they are extremely valuable for forging links between the Upanisads and the rest of the Vedic tradition. At times it takes what seem to be dizzying flights of fancy and so may be described as intoxicating and engrossing. If one were to choose one of the two articles, I would recommend "Sur la notion de Brahman", which traces the evolution of the word and concept brahman with abundant examples and enlightening results. Mystical.
Vishva Bandhu, in collaboration with R. Bhim Dev and A. Nath. Upanisadic Citations
(Upanisaduddharakosa). Hoshiarpur: Vishveshvarananda Vedic Research Institute,1972.
In Sanskrit, and so, seriously limited in its usefulness to most people, however I thought it should probably be included. This reference work is fairly exhaustive, giving citations for thousands of words that occur in the Upanisads, including some of the later ones. It is essentially a concordance. Not as thorough as Vishva Bandhus Veda and Brahman concordances, but those are pretty tough standards to judge by. If you know some Sanskrit and want to do some research, for example, in the ways that a certain word or concept is used in the various Upanisads, you need to use this. If this describes you, ask me (Alex), Kevin or Justin to show you how to go about it: its not just in Sanskrit, its also in a kind of code!