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.