A wealth of documentation can be found on the Google Code homepage: http://code.google.com/p/cocoaasyncsocket/
If you are new to networking, it is recommended you start by reading the Intro page:
http://code.google.com/p/cocoaasyncsocket/wiki/Intro
If you are a seasoned networking professional, with 10+ years of experience writing low-level socket code,
and detailed knowledge of the underlying BSD networking stack, then you can skip the CommonPitfalls page.
Otherwise, it should be considered mandatory reading:
http://code.google.com/p/cocoaasyncsocket/wiki/CommonPitfalls
The API reference page can be found here: http://code.google.com/p/cocoaasyncsocket/wiki/Reference_GCDAsyncSocket
In addition to this, the headers are generally well documented.
If you have any questions you are welcome to post to the CocoaAsyncSocket mailing list:
http://groups.google.com/group/cocoaasyncsocket
The list is archived, and available for browsing online.
You may be able to instantly find the answer you're looking for with a quick search.