Tomcat 6 supports the latest Servlet/JSP specifications i.e. Servlet 2.5, JSP 2.1. Other than that there are a number of changes, such as performance and memory optimizations, faster and more scalable Connectors (such as the 'native' APR Connector and the NIO Connector) and an improved clustering implementation.
Regarding how this book compares to 'Core Servlets', this book doesn�t cover web application development using Tomcat- there are a lot of other good books that fulfill this need. We believe in doing one thing well, and we focus on the administration aspects of Tomcat - Tomcat configuration, performance tuning, system security, and deployment architecture.