AppleScript is an English-like, easy-to-understand scripting language built into every Mac. AppleScript can automate hundreds of AppleScript-able applications, performing tasks both large and small, complex and simple. Learn AppleScript: The Comprehensive Guide to Scripting and Automation on Mac OS X, Third Edition has been completely updated for Mac OS X Snow Leopard. It's all here, with an emphasis on practical information that will help you solve any automation problem―from the most mundane repetitive tasks to highly integrated workflows of complex systems.
An excellent, but very long introduction to AppleScript. It gives a great explanation on all facets of the language, and touches on some of the most robust applications that have AppleScript support as well.