Illustrator reference books?

This is a little off topic, but a few weeks ago, I had gotten two great suggestions for books that have made great reference sources (Scott McCloud's Making Comics, and Bruce Blitz's The Big Book of Cartooning), and I was wondering, what books do ya'll have on your bookshelves that you find yourself constantly referring to?

Personally, I recommend - Writing with Pictures: How to Write and Illustrate Children's Books by Uri Shulevitz. It's very helpful in understanding the process involved in creating a picture book.