There are so many to choose from. >.<
I really love Stephen King novels; they always have this really deep sense of realism, even with supernatural events occurring. ("The Shining" was a masterpiece!)
Paulo Coelho is also a really great author. He tends to go for the philosophical approach, which really makes me wonder. "Brida" was wonderful.
As for graphic novels, Alan Moore wins hands down. "Watchmen" was so interesting, and I can't wait until I get my hands on a copy of "V for Vendetta".
Lastly, for manga, I would have to say CLAMP. That's not really an author, but I really like their work.