AppleScript: The Definitive Guide, 2nd Edition On the tech support bookshelf. More reference than tutorial, covers everything.
Apple Training Series: AppleScript 1-2-3 On the tech support bookshelf. Teaches AppleScript in detail.
Learn AppleScript: The Comprehensive Guide to Scripting and Automation on Mac OS X (Learn (Apress)) Not on my bookshelf, but the authors are highly respected in the AppleScript community.
AppleScript: A Beginner's Guide This is new, so it should be up to date. So far, the one review says it isn't so good. Buyer beware.
AppleScript: The Missing Manual Old (maybe out of date in places) but highly recommended by others.
AppleScript in a Nutshell: A Desktop Quick Reference (In a Nutshell (O'Reilly)) Older title, on the tech support bookshelf.
Riding the Helix Express No scripting, but it's the Classic Helix resource, so I had to link to it.