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.
The deployment descriptor (web.xml) is not Tomcat specific. It is part of the servlet spec and would be the same in any spec compliant container that you use. As Bear mentioned, the servlet spec is the best resource for learning about this file.
There is a link to the servlet spec in my signature.