You can find the topic in "Hibernate in action" or "Java Persistance with hibernate". In "hibernate in action", the chapter 3 and 6 explain the subject. They are called "Mapping Persistent classes" and "Advanced mapping concepts". It's about the way you map classes to records.

He explained that objects are more concrete for us and when it comes to designing object models, we automatically use a more fine grained approach with object as we are moving from object to records.