The book covers Tomcat 4 and 5. 5 is not a complete rewrite of 4. The main difference is that tomcat 5 supports
Servlet 2.4 and
JSP 2.0, while 4 supports Servlet 2.3 and JSP 1.2.
Internally, the servlet container in 5 is more efficient. For example, in 4 each component that needs background processing has its own
thread. In 5 one thread is shared by all the components.
Some background processing tasks include:
1. invalidating expired session objects.
2. reloading an application whose web.xml or one of its classes has changed.
etc...
This is one of the topics "How Tomcat Works" discusses in detail.