Like all open source projects. Tomcat releases fast and often. The books on the shelf will always be a little behind. The combination of the book that you have and the documentation on the Tomcat site should give you idea of what the differences are.
There may very well be a book on Tomcat 5x but by the time you're done reading it, Tomcat 6 might be out.
I have been very pleased with the Wrox publication _Professional Apache Tomcat 5_ - which is the updated version of the book you have. Although the information it covers is undoubtedly available for free on the net, it has been worth it to me to have it all filtered and in one place. Bill