"The Dark Half" by Stephen King

From Amazon.com:

"Creating George Stark was easy. Getting rid of him won't be ...The sparrows are flying again. The idea - unbidden, inexplicable - haunts the edge of Thad Beaumont's mind. Thad should be happy. For years now it is his secret persona 'George Stark', author of super-violent pulp thrillers, who has paid the family bills. But now, Thad is writing seriously again under his own name, and his menacing pseudonym has been buried forever. And yet ...the sparrows are flying again, and something is terribly wrong in Thad Beaumont's world."