The Theater Prop Handbook (Thurston James) and its companion, "The What, Where and When of Thaeter
Props" - also by Thurston James. These are great references and invaluable for new prop people.
A series that I live by is the "Surfaces" (Judy Juracek) books("Surfaces", "Natural Surfaces" and "Soft Surfaces"). They come with CDs that you can reference on your PC, which is a godsend at times. Bascially, they are nothing but various photos of just about anything you would want to paint. Director says, I want a brick wall - go to the section on brick walls - there are dozens of choices- and it allows everyone to be on the same page before the painting starts. Absolutely the best books I've ever invested in.
Charlie