Kurt Vonnegut
One of my favorite writers, who has influenced my own writing immeasurably over the years. His combination of dark humor, idealism, suspicion of authority, and willingness to deal with the fantastical in everyday life (although he has distanced himself from the SF label in the past) line up with my own inclinations in a lot of ways, not to mention the keen sense of life in the clutches of time (or not, as the case may be). Absolutely among the greatest of American writers. Perhaps the reincarnation of Mark Twain and Edgar Allan Poe in one person.
Whenever I have writer’s block, or haven’t been reading for a while, I grab a random Vonnegut book with the knowlege that I will soon either lose the block or remember the need of reading.
Over the past decade or so, Vonnegut has grown much darker and pessimistic, as discussed in this LA Times article on 2005-09-10.