Today I've taken the JPA 2.0 exam and successfully passed it with 83%.
I started reading "Pro JPA 2.0 Mastering the Java Persistence API" by M. Keith and M. Schincariol and then proceeded to the JPA 2.0 FR specification.
But, as usual, one of the most important parts of my preparation was the Enthuware's mock exam simulator. I think it's definitely worth its price, it consists of a great number of very high quality questions which reflects topics you're tested on during your exam. Really - read the specification or some book and do some Enthuware mock questions - it'll be definitely enough to pass this exam.