I personally liked the Boards Simulator Series, and I used 4 out of the 5 books, but only because they were hand-me-downs from my roommate who took Boards the year before. If I were to buy them myself, I would probably just buy the 3 body systems reviews. No good reason why, I guess just to save money! 😀 The 3/5 is probably the most cost effective group, and contains the most relevent questions to the Boards, although you would probably miss out on Biochem and maybe a couple other areas.
I would recommend using the Boards Simulator Series -- It's got TONS of questions, and great explanations. The one thing to keep in mind, though is that the questions are MUCH harder than the actual exam, so don't let them get you down. 🙂