The Classics Pages: Oracles by Andrew Wilson, including a good medium-sized list of 5-4c. oracles. Wilson is the translator of Harry Potter into Ancient Greek, and author of a hysterical page, the benefits of a classical education, on successful people with classics degrees (eg., the lead singer of Coldplay, who alsoand better in my minddates Gwyneth Paltrow).
Amazon. The Oracles of the Ancient World: A Comprehensive Guide by Trevor Curnow, Lecturer in Religion at St. Martin's College, Lancaster (April 2004). An alphabetical dictionary of some 300 oracles in 25 countries. This book has not yet received any reviews online. I'm pitching to get a review copy myself.
Ancient Greek Oracles, a short fact-oriented popular introduction from Archaeonia, an online magazine/encyclopedia of the Greek world by Christos Thomas Vlamis. Vlamis' text is admirably hyperlinked to other sections of the site, but he goes overboard on boldface phrases. Related entries include Apollo.
Encyclopedia Mythica: Oracle by George Hager. Basic.
"Oracles, seers, and prophets in Greek tragedy" by Jonathan, for an AP English class. Uncontroversial thesis: oracles foreshadow.
The history of divination by "the editor of Heretic Press" (not, apparently Patrick Hanrahan). Lengthy grab-bag of true and false information, "Jesus Hoax" arguments, splenetic attacks on "the inner esoteric circle of mental masturbaters who think they are better than others," etc.
Wikipedia Hall of Shame:
"Pythons and Prophecies..." by Joanna Lehmann, from the Fortean Times, "The World of Strange Phenomena." Another detailed, readable, semi-scholarly account, decked out with bibliography and footnotes that is, at the same time, very speculative, even fantastic.
New age and pagan
The Church of The Most High Goddess. Modern pagans with a lot to say about oracles. This is a rather typical melange of authentic facts demonstrating wide reading, autodidac methodology, and hillarious philological and historical gaffes.
Amazon. Have You Been to Delphi? : Tales of the Ancient Oracle for Modern Minds by Roger Lipsey, "retells and reinterprets tales from ancient literature." Includes an afterward, quizzing a Tibetan lama about the Tibetan state oracle.