The following Web sites and books are sources of more information on rootkits:
Understanding Malware: Viruses, Spyware and Rootkits Mark's Microsoft TechEd 2005 webinar covers viruses, spyware, and rootkits.
Mark's June Windows IT Pro Magazine article provides an overview of rootkit technologies and how RootkitRevealer works (subscription required).
This site contains sample code for a number of user-mode and kernel-mode rootkits as well as ongoing discussions on how to develop rootkits.
This site stores the archive of Phrack, a cracker-oriented magazine where developers discuss flaws in security-related products, rootkit techniques, and other malware tricks.
This is the Microsoft Research rootkit home page where Microsoft publishes papers and information on its efforts to combat rootkits.
The Art of Computer Virus Research and Defense, by Peter Szor
Malware: Fighting Malicious Code, by Ed Skoudis and Lenny Zeltser