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.
There you go.