The main documentation resource is the Fawkes Trac Wiki. Some articles of particular relevance:

Fawkes Trac
General documentation wiki, ticket tracking, source code browsing etc.
Fawkes Plugins
Documentation about existing plugins.
How to get started developing with Fawkes.
API Reference
Class documentation for Fawkes.

To get involved and and to get in touch with other Fawkes users and developers please consider the fawkes-devel mailing list.