My final for Illustration concepts, I did portraits of three science fiction writers. The biggest problem with these guys was deciding what age to draw them at (so before you look at one and say "Hey now, he's far too young/old/whatever" take a quick google search around. I assure you, all three gentlemen were once this young/old/whatever.)
First is Douglas Adams, author of Hitchhiker's Guide to the Galaxy.
Second is Isaac Asimov, author of... well, a lot of things. But known for his Foundation series (and Robot series.)
And last is Ray Bradbury, author of Farenheit 451, The Martian Chronicles and more! Mr. Bradbury is my particular favorite, mostly because of his crazy insistence on being a crotchety old man (and those sexy hipster glasses.)