While not a walk through the code base, there are these wonderful volumes: http://www.aosabook.org/en/index.html http://www.aosabook.org/en/index.html that have

While not a walk through the code base, there are these wonderful volumes: http://www.aosabook.org/en/index.html http://www.aosabook.org/en/index.html that have creators/maintainers/contributors walk through at a higher level how these amazing programs work.