So, I should probably preface this by saying that I am a biased author of the book Hibernate Made Easy, so my opinion may be skewed. But here's my two cents.
Every Hibernate development team needs a copy of Java Persistence with Hibernate (JPWH). That book covers everything, from every angle, and will help you put together extremely advanced and complicated applications that leverage everything from advanced locking to design patterns. It's a comprehensive reference, and as a community, we're all very lucky to have a book like that around.
However, when I tried to learn Hibernate, I found JPWH to be a little overwhelming. JPWH has a few negative reviews on amazon, and it doesn't deserve them, but the reason for them is that many readers tried to use this book as a beginners guide, and while it does have many examples, I don't think it's perfect for that.
This frustration is what drove me to write Hibernate Made Easy: Simplified Data Persistence. This book helps someone that is new to Hibernate, but does have some Java and database knowledge, get up and running with Hibernate quickly. It will take a new user and turn them into an intermediate to advanced Hibernate user quickly. The book contains many self-contained examples, so you can open up any chapter and just start coding along - there's no need to download any source code or complete a bunch of code from previous chapters to start learning.
Furthermore, many people that are frustrated by JPWH read my book, really understand how Hibernate works, and how to use Hibernate, and can then jump into JPWH and really understand what Gavin King and Christian Bauer are talking about. And really, that's the key.
My book really simplified Hibernate, and makes learning Hibernate Easy. Part of that is the use of JPA annotations. If you're dedicated to using mapping files instaed of annotations, you might want to look at Harnessing Hibernate. This is a continuation of the Hibernate Developers Handbook, which itself is a good read.
Personally, I really think a developer should have all three books. If you're billing your company, and it's not coming out of your own pocket, I think a library that has JPWH, Harnessing Hibernate and Hibernate Made Easy is a very complete collection that will help you solve most of your Hibernate and Data Persistence related issues.
Here's a little bit more on my book recommendations from my website. There's a number of free Hibernate3 tutorials and code samples there to help you out as well:
http://jpa.ezhibernate.com/Javacode/learn.jsp?tutorial=25hibernatebookreviews
Kindest regards,