Free Book: Mastering EJB 3.0
Published in July 2006, the best selling book Mastering EJB is now in its fourth edition and has been updated for EJB 3.0. This edition features chapters on session beans and message-driven beans, EJB-Java EE integration and advanced persistence concepts. In-depth coverage of the Java Persistence API and using POJO entities with EJB is also included.
This is a great book that explains the workings of enterprise java beans. EJBs often provide a great mechanism accessing resources in another server or access to objects inside the database. If you work in enviroments that use a tiered architecture, you'll find this book useful.