Professional Apache Tomcat 6 / Vivek Chopra, Sing Li, Jeff Genender. Wiley 2007 ISBN: 9780471753612
This is the quickest way for a systems admin to understand Tomcat. There is a lot to learn, and you need to know it all.
This book is not for the Java J2EE programmers who develop the servlets and web applications. Neither is it for a Tomcat developer.