I wrote a book, Linux Kernel Development. It is a fun and practical guide to the design of the new 2.6 kernel, with an eye toward allowing the reader to learn enough to get some real work done.
Look for it in fine book sellers everywhere this coming Monday (8 Sept 2003).
It was a lot of fun, although more work than anyone should have to endure... but the end result was well worth it. I hope readers agree.