Hibernate in Action
Hibernate in Action is a really, really good book. In my previous life as a Python hacker I have spent quite a few evenings hacking together an object/relational mapping framework much like hibernate, only in python. This book starts with a very good, very thorough, very clear introduction to the whole problem of mapping objects to relational databases. The remaining chapters are clear, concise, well-paced and well-written, too. My next book on a Java topic is going to be a Manning book, too.
It's a bit of a pity then that Hibernate is a typical Java library. It comes with a slew of dependencies, it needs ant, but doesn't build out of the box. The readme isn't accurate enough to fix that, you need to go to the faq to find out which libs are missing... Oh, well, I'm going to try to port a Java app I've just finished to Hibernate anyway, to see if it works.