JBoss in Action of course.
Well. It kind of depends on what your role will be and what technologies you'll be using. Will you be administering the application server or will you be developing? What version of JBoss AS will you be using and what version of
Java EE /
J2EE are you using?
There are four main books you'll find on Amazon:
JBoss in Action (Peter Johnson and my book) - This book is good if you'll be administering or developing applications on JBoss 5.0. It doesn't teach you how to develop Java EE, so you'll have to pick that up somewhere else, but it does give you a lot of details on the application server. This book is being printed and will be available shortly.
JBoss at Work - Covers JBoss 4.x and walks you through developing and configuring the application server.
JBoss: A Developer's Notebook - Covers JBoss 4.x and walks through configuring and using the application server. This book is very short but sweet.
I'd recommend any of those three books depending on what you're case is. I wouldn't bother with "JBoss 4.0 - The Official Guide" unless you're planning on actually developing custom JBoss components or you're a developer who wants to learn about the internal architecture of the JBoss 4.0 application server.