The book is very wholistic in its approach to educating the reader. It provides a well-written overview of JavaEE and HTTP in Chapter 2. And whenever it mentions a technology that Tomcat uses in some way, it immediately gives a brief overview of that technology. I found this helpful because it gave me the knowledge I needed to continue reading without having to switch to a search engine. For example, in Chapter 5, when discussing the JNDI functionality that Tomcat comes packaged with, the book pauses for a moment to explain the basic concepts of JNDI itself. The book will also often describe the historical origins of the technology, like in Chapter 1, where it discusses the origins of Ant (spoiler alert: it was created as a custom build system for Tomcat).
The instructions for downloading the Tomcat source code and building it are very clear. It provides a specific branch you can checkout from the Tomcat Subversion server, which is the exact version of Tomcat that was used when writing the book. This allows you to follow along without worrying about dealing with differences between what you see in the code and what you see in the book. And it also includes instructions for getting the project properly configured with Eclipse.
The later chapters of the book go into a lot of detail about the various classes that make up the Tomcat source code, so be prepared for some serious code spelunking. Anyone who is interested in doing serious coding in the Tomcat codebase will gain a lot from these chapters.
I'd recommend this book to people who want to work on the Tomcat source code and need something to guide them through it. I'd also recommend the book to JavaEE developers who use Tomcat on a regular basis and who want to expand their knowledge.