This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori 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.