The International Thesaurus of Quotations by Eugene H. Ehrlich (Editor), Marshall Debruhl (Compiler) More than 16,000 quotations organized by subject, not author. With categories ranging from Ability to Zen, there are plenty to choose from (over 1000 categories). There is also an author listing in the back, just in case you need to look up quotes by author. [More Reference Quotation Books]
Brewer's Dictionary of Phrase and Fable by Adrian Room, Terry Pratchett, John Buchanan-Brown (Introduction) This book provides the reference to the idiomatic sayings in our language. That sounds really boring, but it's incredibly useful. I receive many emails that ask the origin of some phrases such as "the whole nine yards." I'm not able to answer questions like that, but a book like this does, 90% of the time. [More Other Reference Books]