Well I personally just started to mess around in the source,
and the two files I found more important ( I may get really criticized )
in my opinion are DPSrvr.cpp and DPClient.cpp
because there you can find many examples of how source classes are used and even find examples
of certain functions, and from there you could barely go everywhere.
Anyway, you could also take a look at the common files.
But it would also depend what you are planning to do or what you want to do at the moment.