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 It is posted here for historical reference.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 Wednesdays at 10:00 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.
Update: The meeting time has changed. As of Nov. 18, Evolution team meetings are held on Wednesdays at 15:30 UTC.
These are the logs of recent meetings.