The Dark Half is in my opinion one of Stephen King's best works. I've read several of his books over the years. I must admit that I lost interest in his work for a long stretch of years after high school. However, this novel is intensely creepy and a blazing page turner. Thad, the protagonist, is a writer who finds himself at odds with his old pseudonym, George Stark. Thad's works aren't that successful, but when he writes under George's name he produces great work about a violent killer named Alexis Machine.
George comes to life and Thad struggles against him throughout a large part of the story. Supposedly, King wrote the Dark Half shortly after he was found out to be Richard Bachman - ironic!
This is an old book (around 1989), but definitely worth checking out.