Rishi -
I'd say that if you made up a long detailed list of "topics covered", you would discover that the 3 books overlap a lot. In the last few days there have been quite a few posts on this topic, so you might want to look those over, but in a nutshell, HF EJB has two goals (Kathy might "update" my list
),
1- An intro to EJBs. In general, the idea of a Head First book is to focus on the key concepts that are necessary to really GET a technology. We purposefully leave out less important topics so that we can focus on the key stuff. We want our readers to finish our books feeling like they really understand the topic. Maybe not ALL the details, but the core, important ideas. If you really get the basics, you can pick up the details easily as you need them. (All the pro athletes constantly return to the fundamentals season after season!)
2 - An SCBCD certification guide. Normally we prefer not to have two goals, but our feeling is that the SCBCD exam does a pretty good job of focusing on the same topics that we feel are fundamental, so in this case we thought we could do a good job of supporting both goals.