A good place to start is Apple’s own books (PDF’s) on Applescript. They’re fairly old, but the Language Guide is still considered the “bible” of Applescript.
Apple has a page with:
AppleScript Scripting (an overview)
AppleScript Language Guide
The AppleScript Finder Guide (How to use scripts to do Finder tasks. It’s old, but useful still)
Now that Mac OS X 10.5 Leopard is out, Apple has released (finally) a new version of the Applescript Language Guide. The old version (see above post) goes all the way back to Applescript 1.3.7 (OS 8.6). This new guide is written with Leopard’s Applescript version 2.0 in mind, and so while it may be helpful to Tiger or Panther users, there will also be items in the guide that apply only to Leopard’s Applescript.
In addition, there is a new link for Apple’s Scripting & Automation guides.