This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I like the Wrox press book "Professional Apache Tomcat 5" for understanding how to get Tomcat to do what you want. For understanding the rules that Tomcat follows, download the servlet API. For understanding exactly how Tomcat functions internally, I think you are stuck with reading the actual source code.