I agree with Mark that the EJB 3 spec is very well written, and that it is written for a broad audience that includes app server providers (EJB container implementors) as well as EJB developers. By necessity the spec covers every intricate detail that can be specified. We aim our book squarely at the the EJB (and JPA) developer who is coming upon EJB 3 and looking for a convenient on-ramp to the techology. We elaborate on the main areas of the spec, while filtering out a lot of the advanced details, and reduce it to the most important concepts we think most EJB developers will need to get started.
Part of the trouble with a very broad technology like EJB is there is a lot of material to cover before you can see the trees for the forest. It is very difficult to jump into the advanced details of EJB and JPA before you have a good understanding of the most important concepts. Instead of wading into these advanced concepts, in many places we point readers to the appropriate area of the spec if they are interested in more depth. This way we are able to cover a lot of topics and keep the pace moving forward, without getting bogged down in the many advanced complexities that are covered in the spec documents.