Tanenbaum's OS book is superb. One of the few undergrad CS books I've kept (most of the rest were on the math side of things; while I think that's cool, it's not so much for me to keep...)

I know of no book in print today about how operating systems work. Sadly, there are only books about how Unix (and sometimes VMS, in its NT incarnation) works.

The fact that the two subjects are seen as one and the same is why systems innovation is all but dead:

http://www.eng.uwaterloo.ca/~ejones/writing/systemsresearch....

"Understanding the Linux Kernel" from o'reilly goes into a surprising amount of detail not only from the OS's point of view, but also the architecture it's running on and the memory management unit. It's approachable and is not light on detail.

edit: email me if you have any specific questions.

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-...

Checked some the blog posts, and they are good. Are the blog moved to some other place. There is no activity from feb09.

http://williamstallings.com/Extras/OS-Notes/notes.html is a cool notes for learning OS