I have around a year's experience in EJB3. I just had an eye on EJB in action for understanding few concepts and thoroughly gone through MZ guide. I didn't find much difficulty in understanding MZ guide. I am not sure whether I am prepared for the exams or not. Do I still need to go through Mastering EJB3 and EJB in Action thoroughly?? is MZ guide enough for 310-091? I am yet to go through mock exams.
Try some mock exams and you will know where you stand. I have used Enthuware and it was really a great learning tool. If you get 70+ in Enthuware mocks, you will definitely get an excellent score in the real exam.
Make sure you covered transactions and security pretty well. You will get a lot of questions in these two. Take a look at the EJB3 specs where it covers these topics. Also skim through the JPA specs.