The Architecture of Open Source Applications

http://www.aosabook.org/en/index.html

Hey, at least it's free.

>>8
It's size is fucking enormous:
Wesnoth's core engine is written in C++, totalling around 200,000 lines at the time of this publication. The game ships with another 250,000 lines of WML code.

You probably can compress it by an order of magnitude (down to 20,000-30,000 lines), but you'll still've to read through whole C++ codebase.