If your interest lies in the realm of inherited diseases, you'll find that Online Mendelian Inheriance in Man is an absolute goldmine of information.
New England Journal of Medicine may be useful in various ways (some full text, search engine), and JAMA offers text of back issues and a search engine (mid-1995 on).
Some specific diseases:
You should be aware of Handbook of physiology (Science REF QP6 .H25): more than 30 volumes of basic information. Also take a look at Textbook of Medical Physiology (Science REF QP34.5 .G9 1986). And of course The Merck Manual (Science REF RM127).