Getulio,
Everyone is different, but at least when I took part I, I felt that the
test was a bit harder than the Whizlabs simulator. I even think I scored lower on the real test than on my last runs on the simulator. On other Oracle tests I've taken it has normally been the opposite. The OCMJEA test has some very subjective questions, which make it harder in my opinion. When I did the test there was no book for the version 6 of the test, but if there had been one, I probably would have read it cover to cover.
Good luck,
Andres