Part 2 exam, in my opinion was better designed than part I, there are some questions that are really good, but there are some too that you can just guess. Most questions are quite clinical oriented, and or theory based applied on clinical settings.
Dental material is a major part, and there are some questions with basic statistics. If you have learned the basics of statistics, you don't need to go through the books to review them all.
The way I studied for NBDE was that I started from question papers, I did the questions, mark those that I have doubt with and then study " extensively" on the relevant topics of the questions that I am not sure about. THere are very few repetitive quetsions in my part II exam, but the old exams really helped a great deal, they guide you to the field that you are suppose to cover, if you just memorize down the answers, you will find the questions are similar in the exam, but they are DIFFERENT!!! so do pay attention to old questions, for example, if the old question asked about what kind of congenital disease could cause this kind of sign on X-ray, you should go through all the congenital diseases that would have dental/ oral signs. THe similarities and differences between them, the differential diagnosis etc...
you also " NEED" to have some kind of references, but I personally think it is a waste of time to study the entire book " for the exam" the textbooks are great to study on, but they are not designed for exams, so you may ignore the important things asked in exams if you study the entire book. But You do need some type of references, textbooks, so that you can look them up when you are studying extensively on the questions. In other words, you should be creative in terms of studying for exam,.....when a question ask about one thing, try to imagine all the related stuff, anything that can have something to do with this topic asked, make sure you are familiar with them or check them up!!!
I felt that was my most efficient way of studying to get a good score in a short time.
I studied for a month, got 95 on average.
I was a full time student at the time, though I had graduated from dental school already
PS, I took computer based exam, and I definitely recommend it
hope it helps