I'd think that Tanenbaum's book Operating Systems: Design and Implementation (OSDI) and the included MINIX3 source would be high on the list of reading for any kernel-curious developers.
Good, but a little outdated. For general systems overview, as well as introduction to UNIX architecture, I'd actually recommend my textbook, which is the CMU intro systems book, Computer Systems: A Programmer's Perspective[1]. For OS theory, our OS/advanced OS textbooks are fine, but for actual implementation my coworkers recommended The Design of the UNIX Operating System[2], Linux System Programming[3], and Understanding the Linux Kernel[4].
Minix 3 is a whole different ball game though. It's an interesting kernel with some interesting ideas, but I still recommend Linux as it's both more popular and reflects the majority of UNIX design decisions today. Even the OSs with praise for microkernel design tend to incorporate a number of monolithic features.
EDIT: Sorry! I had no idea there was a Third Edition of OSDI out http://www.pearsonhighered.com/educator/academic/product/0,,.... However, I should mention that I meant those Linux books to be read in conjunction with the kernel code -- the book is still valid today despite the additions to the kernel in the past couple years.
