I highly recommend "Operating Systems: Design & Implementation" by Tanenbaum. It is a very clear book dealing with a lot of design and implementation details of

I highly recommend "Operating Systems: Design & Implementation" by Tanenbaum. It is a very clear book dealing with a lot of design and implementation details of operating systems. It has an in depth discussion of the Minix source and design decisions made. Now Minix does not have such a large user base as Linux, but in my kernel hacking experience in both Minix and Linux so far the Minix sources are far more pleasant to hack on.