This is Hibernate's Migration Guide from 2.x to 3.x.
http://www.hibernate.org/250.html
I have bought the book "Pro Hibernate 3". However, it doesn't go into much detail, but it was good to get the general feel for the new syntax. And there really isn't much in it that you don't already get in the Hibernate docs that are downloaded with Hibernate 3.0.
If you just punch these URLs in your browser you can see the DTDs.
http://hibernate.sourceforge.net/hibern ... ng-2.0.dtd
http://hibernate.sourceforge.net/hibern ... ng-3.0.dtd
Brian