I'll mention this only once, promise.
The third edition of my best-selling Linux kernel primer, Linux Kernel Development, is now shipping. The publisher is Addison-Wesley and you can find the book in better local and online booksellers.
What's new? Lots. The text is clarified, reworked, and reorganized. The material is fully updated and now based on the 2.6.34 kernel. I also note if and how the 2.6.32 kernel differs, as that release is a "long-term stable" version and thus likely relevant to many readers.
New material includes treatment of the Completely Fair Scheduler, an analysis of the flusher threads component to page writeback, and a chapter devoted to kernel data structures.
Whether you already own an earlier edition, whether you are a kernel developer or just curious how it all works, Linux Kernel Development is an excellent addition to your bookshelf. But don't take my word for it. From one of the many five-star Amazon reviews (this one of the second edition):
The content is one part why this book is great, but I think bigger points go to Robert Love's delivery. His style is casual yet not willy nilly. A subject like the kernel is both dense and minutia-filled and in turn is so much easier to grasp when it's not presented in a dry academic style. The way I would characterize this book—it's as if you're older brother took you aside and taught you how to hack the mainframe.
I have been doing Linux kernel/system level development on and off since 1999. This is the book that I think should be owned by any Linux newbie who wants [to get into] kernel hacking. Even if people do not directly do Linux kernel development, it is a good book complementary to any serious operating systems course in college—it helps gain a better idea of how and why.