Don't buy books over summer to "pre-study", spend your next few months NOT thinking about learning, it will hit you hard and fast, and unless you plan on taking didactice year twice, you can't really prepare for this first year.
If anything, get your life in order to a meticulous level, so you don't have anything to worry about come the first day of class.
Ask previous students about their opinions about which books were needed and which weren't (based on the book list you should have gotten from the program)
If you do decided to attempt to prepare, the only thing that is a standard "need" is to study anatomy, it all comes back to anatomy...and if you know it cold, then you can spend your efforts on other areas once study time is precious.