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 It is posted here for historical reference.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.
Update: It was decided in the June 20, 2012 meeting to end regularly scheduled team meetings on account of the Evolution team having so few developers left. The remaining developers will use the evolution-hackers mailing list as the primary forum for technical discussions, status updates and upcoming event reminders; which has already been the case in practice for some time.