Hibernate is an Object/Relational mapping framework that attempts to bridge the gap between Object-Oriented programming model and relational database model. In the past, many attempts have been made to bridge this gap or replace one with another, but the gap between the two is one of the hard facts of enterprise computing today. Hibernate is one such attempt that enjoys a lot of success in the community today.
Hibernate in Action, authored by "Christian Bauer" and "Gavin King" (Gavin, being the creator of hibernate), is one of the best books on this topic. Traditionally we have witnessed that the best book on a particular technology is not written by the creator of the technology themselves, but by authors who have the proficiency to explain it pragmatically. Gavin, breaks the shackles and contributes the best book for the technology he created.
The primary target audience is java developers who work with SQL based database systems and have struggled hard to bridge the gap themselves.
The book starts by explaining what "Object/Relational paradigm mismatch" is all about. The authors weigh this to be the prime motivation for any ORM implementation, and wish that the users understand this mismatch, before they jump-in believing that ORM is the magic bullet for all their problems.
The chapters "Introducing and Integrating hibernate", "Mapping persistent classes", "Working with persistent objects" and "Transactions, concurrency, and caching" introduces the basics for various aspects of hibernate. These chapters are the most beneficial part of the book and well exceed the book's cost.
The chapters "Advanced mapping concepts", "Retrieving objects efficiently", "Writing hibernate applications" and "Using the toolset" focus on the advanced concepts. These chapters are rather dry and are mostly useful for reference purposes only. So, if you get lost while reading these chapters, don't worry. It covers so many advanced concepts that not everything will fit into one's head in the first pass. If you frequently visit this section for reference while developing hibernate applications, you will start to appreciate this section slowly but steadily.
In my opinion, if you want just one book to learn and use hibernate for your next project, look no further.
But, don't forget, the next edition is just round the corner. It covers Hibernate 3.x and also includes full coverage of the EJB 3.0's sub specification called Java Persistence API (JPA). So, if you are not in any urgency, then you might want to wait, otherwise, go ahead and get this one for your bookshelf as soon as you can. You won't regret it.