FileMaker 9 Developer Reference. Functions, Scripts, Commands and Grammars with Extensive Custom Function Examples.
Que publishing. By Bob Bowers, Steve Land and Scott Love.
And, if you can get by the typos and mis-referenced examples you might want to get:
FileMaker Pro 10 The Missing Manual. It is clearly written, (usually) and covers a lot of what is on this board.
By: Susan Prosser & Geoff Coffey
I found these books with this board to be of immense help. Good luck!