Wasn't the character of Sherlock Homes inspired by one of ACD's professors at medical school? Apparently he could deduce many details about a person by subtle physical signs.

I believe the following writers were physicians:

William Carlos Williams
Walker Percy (one of my favorite writers)
Michael Crighton

Then there's Oliver Sacks, who is a practicing neurologist who writes what I would call medical books for lay people. The late Graham Chapman from Monty Python's Flying Circus was a medical school graduate. I am not sure if he ever actually practiced or if he simply went straight into clicking coconuts together.