Gustavo Duarte's blog posts are excellent. Clear, readable and detailed explanations of things like what actually happens when booting up and how memory management works. He links to relevant parts of the Linux source code for added context, but also covers how other OSes differ from Linux, and covers how OS design and CPU architecture work together.

http://duartes.org/gustavo/blog/

http://duartes.org/gustavo/blog/post/kernel-boot-process

http://duartes.org/gustavo/blog/post/how-the-kernel-manages-...