My favorite author is Kurt Vonnegut, and his books are written in a style that is pretty difficult to adapt to film. the only decent film adaptation of his work is Slaughterhouse Five, and that is just an ok film that doesn't quite capture the magic of the book, one I consider one of Vonnegut's weaker efforts. By all accounts, the only two other adaptations of his novels I know of, "Harrison Bergeron" and "Breakfast of Champions" are both considered total failures. To be honest, I'd rather Hollywood just left him alone.
I'm also very fond of Cormac McCarthy, his best book (and my favorite) by far is the sickeningly brutal western "Blood Meridian". Supposedly after the successes of adapations of McCarthy's The Road and No Country for Old Men, a film adaptation of Blood Meridian is in early stages of development. I'm torn because if handled correctly, it has the potential to be the greatest thing ever...but if they fuck it up, I will be pretty pissed. Casting will be essential for that one, if they don't get exactly the right people to play The Kid and The Judge it will be dead in the water.
Much riper for cinematic adaptation are graphic novels. Seeing how awesome of a job was done with stuff like Watchmen/Sin City and The Walking Dead series, I'd love to see someone take a serious crack at adapting either Neil Gaiman's Sandman series or Garth Ennis' Preacher series. Both of them are absolutely fantastic books/comics and would be incredibly well suited to a weekly series (though a film/film series would be even better).