Sticky threads in the http://www.quake3world.com/forums/ programming section.
http://www.quake3world.com/cgi-bin/forumdisplay.cgi?action=topics&forum=Programming+Discussion&number=4&DaysPrune=10&LastLogin=
This http://www.linux.ucla.edu/~phaethon/q3mc/q3mc.html is a nice overview of q3 game execution path and data structures.
A doxygen crossrefrence, with additional comments: http://www.soclose.de/q3doc/
I suggest any new ET modders take a look. While all the above is for Q3 and Q3TA, the basic organization of the code is the same, as are many functions/data structures etc.