Just wondering is it worth to buy HEAD FIRST EJB 2.0 to understand the EJB world and clear the basics , and then jump on these books?
Mmmhh, no, you'd better not. Go for EJB3 In Action instead. You'll see it's easy to go through.
How different is EJB 3 is from EJB 2?
Although the logic looks very similar (expect for Entities), there's a lot of of new things to know. Don't use an EJB2 book. If you're interested in EJB2.x, study them AFTER the exam