No single source for an exam as important as this should be used, instead at least three sources should be relied upon. The M5 is a great foundation for learning material as well as assessing strengths and weaknesses. In addition to this, you should have a solid review book (M&M, etc) and one reference book (Miller, Barash, or Longnecker). There are no question-based books that have the depth, comprehensiveness, or quality of explanations comparable to the M5 that could be used as a foundation; but, that doesn’t mean they wouldn’t be helpful adjuncts. Focus on taking this exam once!