People with Medical degrees.

Sometimes I think it is the pharmaceutical companies for the first thing doctors do is to push on me the pills these companies sell.

Doctors who have loads of experience in their field. Nearly all doctor specialise in a medical area and then they can become consultants and i suppose they are asked to write. Thats what my bro said and he is a trainee doc.

Quite frequently there are a large number of contributors that help to compile a textbook. In the best text books there maybe a principal editor and/or a principal author who will dictate the overall tone of the book. Usually the most recent research at time of writing will be consensed into textbook form either by the author/editorial team or a specialist in the area will be contracted to do it. So quite frequently different teams will work on different chapters. All this is if you're talking a broad-spectrum textbook. If you're talking about a more narrow textbook (i.e. specialist for one area of medicine) then usually the author and editor will be specialist in that area already and are able to write without too many other contributors. If this is done well and the textbook is good it is likely to become more widely used. If it is badly done and the textbook is poor, it might still get published but not be widely used. Here's a great example. Anybody who has ever done anything with an aspect of biochemistry will at some point have bought, borrowed or stolen a copy of Stryer Biochemistry. It's a very well assembled textbook, frequently updated, with excellent contributions. I don't think I've ever met anyone who has studied any biochemistry in depth who hasn't seen Stryer.