You'd better read the entire EJB 3 in Action book. But that alone would not be enough for the exam. For a through understanding on EJB Entities. use Pro EJb 3 and later use Enthuware or Whizlabs for doing some practice tests. May be if you have time, skim through the specs as well.
SCJP 1.4, SCWCD 1.4 - Hints for you, Certified Scrum Master
Did a rm -R / to find out that I lost my entire Linux installation!