This is a very well written book about ActiveMQ. I may be using ActiveMQ on an upcoming project, but I wanted to learn about more about enterprise message queues in general. I would recommend this book for that purpose because it is helpful to have concrete examples as opposed to general statements about what some message queues can do. The good thing (or maybe bad, from a complexity standpoint) about ActiveMQ is that it can be configured to work in many different ways. So, after reading this book you should be knowledgeable enough about message queue features to evaluate and use other message queues.
The only thing I would like to see is an updated version. It is approaching 3 years since the original publish date and, as is the nature for technical books, some things have changed. Most of the code samples work, but for some things it is worth searching for more up-to-date examples.