If I’m not wrong to assume that a book that’s primarily on Apache’s web servlet Tomcat project should have “Tomcat” in the title, this list of books and their publication years suggests that nobody’s writing new books or updating their old ones.  By the way, I’m using Tomcat 8, for where there seem to be no books.  Even Amazon.com has no English-language books with “Tomcat 8” as part of its title.

Beginning JSP, JSF and Tomcat: Java Web Development
2012

Apache Tomcat 7 Essentials
2012

Apache Tomcat 7
2011

Tomcat 6 Developer’s Guide
2009

Tomcat: The Definitive Guide, 2nd Edition
2007

Professional Apache Tomcat 6
2007

How Tomcat Works: A Guide to Developing Your Own Java Servlet Container
2004