As far as I remember, joining the AMA gets you scrubs, Stedman's medical dictionary and Rohen's atlas. Rohen's has actual photographs of cadaver dissections and such. An AMA membership costs in the neighborhood of $80 for 4 years (if memory serves). Anyway, you will be contacted shortly by the Wash U chapter with details.
A membership with AMSA comes with a Netters atlas, but you have to fill out a credit card application (you have no obligation to actually open an account though, you only have to fll out the application). I took a membership with the AMA and then managed to get Netter's from the barnes and noble website since they were giving a 20% discount or something. Both are good atlases; I ended up using Netter's a lot more.
As far as other books go, this will vary from person to person. You will need a Grant's dissector for anatomy. Most people (myself included) just studied from class notes which are quite comprehensive. Some people did study from textbooks--Big Moore and Little Moore being the most popular. Little Moore is definitely more readable, but Big Moore has a lot more detail. Both are good references.
Course notes suffice for most other courses. You may want to buy Lippincott's biochem, especially if you have never taken a biochem course before. But again, the course notes will work well for that course. BRS Physio and/or Physiology by Costanzo are both great books. BRS is authored by Costanzo, but is more concise. Its definiely a great review.
You will probably need an atlast for histology, although the online Menton collection is a great resource. You guys should be reciieving the Dis-O guide shortly. That will have a better list of books you should buy. I would wait till you get to med school, because a lot of upper classmen will be selling their books in the first few weeks of august. However, the books do sell out really quickly. Buying used books online (half.com, etc.) is also a great option.
Hope this gives you atleast a general idea. Let me know if you have any other questions!