There is information on Git, a short Architecture Overview and a Planning Page. There is also Patch Submission Information. In addition, the links below are quite useful for discovering more information.
These are the docs in their current state
This is the reference manual for the last public release of the GNOME Applications Library (GAL) before it was subsumed by Evolution. It is posted here for historical reference. Much of the library is still used in Evolution.
The Evolution team gets together on the third Wednesday of each month at 15:30 UTC for a status meeting on IRC. Anyone is welcome to attend. The meetings are held in #evolution-meet on irc.gimp.org, and are usually about a half-hour long.
These are the logs of recent meetings.