This is a very good book on performance tuning in general and JBoss in particular. The author shows how to methodically build your performance tuning methodology, and spells out in detail how to test each layer from the operating system through the application server all the way into the application components. The tools used are all open source and well established, so the reader will be able to follow along each step of the way.
The first chapter and a half are a little weak, technically. The rest of the book is beyond full-strength, though.
I'd strongly recommend this book to anyone using the excellent JBoss application server, and would even recommend it to Java application owners in general (Tomcat users in particular can benefit, but not nearly as much as JBoss users.) This is the best contemporary Java tuning book I've seen.
A more detailed review can be found here.
The book can be viewed here.