If you're going to spend the time to learn, you should learn the new Java EE 5 spec and not the old J2EE spec.
JBoss in Action does not teach you Java EE. You probably want to buy a book like EJB3 in Action. Though, you might find it useful to pick up a book that teaches you the specifics of the application server you are going to work with, so if you're using JBoss 5, JBoss in Action might come in handy. If you're going to work with JBoss 4, then you could look into JBoss at Work or JBoss: A Developers Journal.
No and yes. One thing you will find out as you learn Java EE is that the spec does not cover everything and leaves many things up to the application server. And each application server does these additional things in different ways. Our book primarily focuses on those "additional things." Thus we cover the extra configuration you need to make to a Java EE application to get it to run on JBoss AS.
Having said that, we provide very simple applications that we use to describe the extra configuration. You can use those apps as a primer for those topics. But we do not go into depth on any of technologies. For that, you really need other books (no single book I know of covers everything, Java EE is too large for that).
Thanks for your answers.
For the moment, my aim is not to cover all J2EE.
It’s just to discover it as quickly as possible (and to discover how to make it works with a well known plateform) to get an idea on what I can do with it, to later have the pleasure to go in deep with J2EE.