CocoaScripting wraps and dispatches AppleScript commands to the appropriate places in your application, and collects the results and sends them back to the AppleScript engine.
Apple provides some starting points at http://developer.apple.com/referencelibrary/GettingStarted/GS_AppleScript/index.html.
In Mac OS X 10.2 you can use the NSAppleScript class to compile and execute scripts. One can also make AppleEvents programmatically in Cocoa via NSAppleEventDescriptor, which can then be sent to an NSAppleScript object or other apps. See Apple's sample code at http://developer.apple.com/samplecode/AttachAScript/index.html for example.
The rest of the page is mainly on implementing scriptablity in a Cocoa app.
The AppleScript Language Guide:
Check out the developer docs at http://developer.apple.com/documentation/Cocoa/Conceptual/Scriptability/index.html or you can find them as part of your local developer documentation:
[Jaguar] file://Developer/Documentation/Cocoa/TasksAndConcepts/ProgrammingTopics/Scriptability/ [Panther] file:///Developer/Documentation/Cocoa/Conceptual/Scriptability/index.html [Xcode] file:///Developer/ADC%20Reference%20Library/documentation/Cocoa/Conceptual/Scriptability/index.html
Among other resources, here's an old Applescript-in-Cocoa article in MacTech:
Here's a recent article at the O'Reilly Mac Devcenter:
http://www.oreillynet.com/pub/a/mac/2002/02/22/applescript.html Integrating AppleScript and Cocoa
or another one here (KFAppleScriptHandlerAdditions): http://homepage.mac.com/kenferry/software.html#KFAppleScript
Check out AppleScriptingCocoaApp for a brief guide to implementing AppleScript support in your app.
Also check out HowToSupportAppleScript for a more extensive guide.
AppleScript is leaky... NSAppleScriptMemoryLeak
"The Development of AppleScript" (W. Cook, 2006)  is a recent paper by one of the original AppleScript designers that provides some excellent insights into the early history and original objectives and design of AppleScript and the OpenScriptingArchitecture.