If you're talking about the MA program, we don't really use textbooks, just use the prof's syllabus for each class. And you can't get those till you start. Also, the courses are "as difficult" as medical school courses (although I find them fairly easy given that we don't take a full load, really), so that's the level you should anticipate. No reading ahead will really help, but if you must, buy Lippincott's Biochem book and read the parts on amino acids, proteins, enzymes, metabolism, carbohydrates, fats, cholesterol, nucleotides, DNA, and RNA. Then you'll have done the biochem course. Skip vitamins. Memorize everything.
If you can, I would recommend heading to a biomed library or something of that nature if you truly are eager to read ahead. I made the mistake of buying the biochem text only to never open it. Also, the med library on campus has all of the textbooks referenced by the profs. You can just check it out from the front desk for a certain number of hours and make copies or read whatever you like, then you return it before you leave. Honestly, the syllabus of each course is the bible. If you can memorize those then you are golden for each course.