"Dictionary of Theories," edited by Jennifer Bothamley, is one of the best references I've purchased in recent years. As a fiction writer, I've collected many compendia of information and this one has proven to be one of the most used. What is the "Pleasure Principle"? What are Aristotle's "Four Causes"? What about the "Big Bang Theory"? The "People" and "Subject Area" indexes at the end of the book, along with the bibliography, are very helpful and make this dictionary that much more fun to use.