Buying books ahead of time is stupid. Why because you will save more money by not buying the books you will never use. I only bought one text book for MSI and I even regret buying that one because I never use it. The money is better spent on review books but check them out in your school library first to see if you actually like them.
I survive on scribe notes and power points. Any concept I don't understand I look up on the web.
You won't know what is really "required" versus "recommended" versus a total waste of money, until you get to school and talk to upper classmen. So I suggest buying nothing unless you actually have a reading assignment for day one. However some schools have been known to do a bad job at ordering texts through their bookstore, resulting in shortages, so there's a chance either you will have to muddle through with the class notes for a while while the school gets its stock, or resort to Amazon, if something is truly required. In most cases, the course notes will suffice to do well in the class, and most of the important books will be in the library if you really need it.
If you want to get really cheap, older editions of the books so you can have them as a reference, buying them early is helpful because it gives you time to buy them online and get them shipped. I personally didn't use my textbooks more than a few times my first year (with the notable exception of anatomy), so I was glad that I bought the previous editions. The information is mostly the same as the latest edition, and certainly the concepts are. It's usually just a few new figures or a paragraph or something is added to the chapter so they can sell a new book. Usually you can get the second most recent edition for $5-$15 if you look on Ebay, half.com, and Amazon. If your school gives good lecture notes, you might not ever use your books. If you used your books a lot in undergrad, you might be glad you have them.
In retrospect, the only books I would have bought would be Netter's and Rohen's Atlases for anatomy. Those I used A LOT.
edit: and BRS Physiology. That helped a lot and I plan to use it for the boards anyway. Oh, and Mosby's. That was useful.
Contact your big sib or find some upper-classmen troaling around on the Allo-class threads and ask. Like many on here, I was more of a .ppt and scribe guy. If I had a book, it was a BRS/NMS/rediculously simple/etc. I ended up buying about 1/3 of the 'required' texts over my MS1/MS2 and maybe used half of them..... of that half, only and handfull were actually helpful (as in time well spent). I'm currently selling ALL of them and many in BSN condition
The only books I've found to be essential so far are a Netter's atlas and an atlas for histology. For everything else, scribes and lectures were sufficient and much more high yield than reading through textbooks would have been.