While not a walk through the code base, there are these wonderful volumes: <a href="http:&#x2F;&#x2F;www.aosabook.org&#x2F;en&#x2F;index.html" rel="nofollow">http:&#x2F;&#x2F;www.aosabook.org&#x2F;en&#x2F;index.html</a> that have creators&#x2F;maintainers&#x2F;contributors walk through at a higher level how these amazing programs work.