I've been doing CF for a while now, but there is one book which stays by side through development, ColdFusion 5 Language Reference by Ben Forta. Though it doesn't get much use now having it there has saved me loads of time, and its as i reached for it today i thought about how indispensable it is. Its just the ultimate quick reference too look up a tag or a function attribute, plus it's small enough to take anywhere with me. I'm hoping one day Ben can find time to update the book for CFMX. The other one i try and keep near me is Advanced ColdFusion MX Application Development , which always handing when doing other bits and peices.
So what book, whether used a lot or not, do you like having by your side?