You can find it in a couple of places...
First, and foremost, pick up O'Reilly's EXCELLENT 'AppleScript In A Nutshell'. This book is, from a long-time AppleScript user, GOLD. Even I learned a few new tricks from it, and it makes an excellent reference manual.
There is also MacScripter.net. A great jump-off point for explorations into AppleScript.
MacTech also has some fairly Technical articles on it's use.
And finally, check out Bill Briggs tutorials over at Maccentral.com.