Linux Core Kernel Commentary by Scott Maxwell. Coriolis Group, 1999.
Linux Device Drivers, 2nd Edition by Alessandro Rubini and Jonathan Corbet. O'Reilly & Associates, 2001. Also available on-line at: www.xml.com/ldd/chapter/book/index.html.
Understanding the Linux Kernel by Daniel P. Bovet and Marco Cesati. O'Reilly & Associates, 2000.
The Kernel Newbies Project provides a lot of documentation, the latest versions of the in-kernel documentation, good FAQs, lists of other recommended books, and a good mailing list and IRC channel for people of all skill levels: www.kernelnewbies.org.
“Linux 2.4 Kernel Internals” by Tigran Aivazian: www.tldp.org/LDP/lki/index.html.