TFS:
The most self documenting code is at luascript.cpp. Must read for both new and advanced scripters.
enums.h - holds some of constants used widely in sources
protocolgame.cpp is where client-server structure is processed
game.cpp - lots of game world related stuff
player.cpp - the in-game things related to player
monster.cpp - basic monster ai
map.cpp - tiles and positions, checking for projectiles, pathfinding
combat.cpp, spells.cpp - combat system
cpp files named like folders in data - loading and processing these events
Will skip explaining otclient as I haven't got into coding it much.
If you need to find where the functions go/come from, see this:
useful tricks in Visual Studio for beginners...