Have you tried the Books store in the Apple store in Itunes? Otherwise other than that, don't think there will be much free ebooks on Medical stuff around. If you want the best medical ebook around you should be prepared to pay a little for it. Depending on the medical ebook you want.
I agree with Shadowlad-in my experience with medical books, none were free and were very expensive. I haven't purchased any medical ebooks, but know that it is out there. Have heard that the price is perhaps slightly less expensive than the hardcopy.
Now there are some "consumer" type medical reference books that may be free for the public. It really depends on the topic and type of "medical" information you need.