• The first section 13.1. About Hibernate Core contains only 1 sentence and refers to the second and third sections for more details: 13.2.2. Hibernate EntityManager and 13.2.1. About JPA. However, the third section 13.2.2. Hibernate EntityManager refers back to the first one (plus the second one 13.2.1. About JPA).
• 13.2.2. Hibernate EntityManager contains only 2 sentences, which were already present in previous section, plus a reference to the previous section.

The circular references together with the fact that 2 of 3 sections do not contain much text and the text is repeating is pretty confusing and not much informative. I'd suggest to rework the beginning of this chapter to include all the information it provides now, but more clearly.

Another issues in the beginning of the chapter, which may be outdated after the fix, but I will mention them here:

• Hibernate EntityManager implements the programming interfaces and life-cycle rules defined by the JPA 2.0 specification. - it should be 2.1 specification. However, the link to the specification is already provided in previous section, so it is not needed to repeat it.
• For more information about Java Persistence or Hibernate, refer to the and About JPA and About Hibernate Core. - some kind of typo or copy/paste error: "refer to the and"