This small bible from Zilog clearly lists all the instructions and their definitions - definitely a must have. The latest version can be downloaded from www.zilog.com
You can't go wrong with an Osbourne & McGraw-Hill. All you need to know is clearly laid out and they always give lots of good code snippets.
Not that comprehensive on the language itself but does tell you how to get code into the machine. Peek and Poke away.
A useful learn as you go book