The authors I follow:

Margaret Atwood: @MargaretAtwood
Stephen Fry: @stephenfry
Neil Gaiman: @neilhimself
John Green: @realjohngreen
Joyce Carol Oates: @joycecaroloates
Mary Roach: @Mary_roach

If you’re looking for authors to follow, here are some other places to check:

Top Ten Most Prolific Authors on Twitter from the Telegraph
10 Awesome Authors on Twitter from Barnes and Noble.com
This one is from 2009, but it has over 100 authors so it’s still worth a look.