Merck

When I was in medical school, the Merck Manual was one of the books we often used to look up various diseases as they had a pocket sized version that fit in our coat pockets. Now, the entire home edition of the famed Merck Manual is available online using this link;
http://www.merck.com/pubs/
The reason I will often recommend patients to look up diseases in this format rather than on more commercial sites is that Merck is a pharmaceutical company and they do not focus on "upselling" the consumer or linking to other "paid" advertisements. They give you just the simple facts on the medical diseases and you can rest assured that physicians also use the Merck Manual as a reference tool.