Does anyone know of any books with a main character that has a split personality?
Fight club immediately springs to mind.
Any others you can think of, especially if they have a big reveal at the end?
I'd love to see how people have handled it.
For me, the obvious 20th Century classic would be The Atrocity Exhibition by J. G. Ballard.
"We Are Glass is a bruising encounter yet it flickers with compassion and is the best short story collection I’ve read since Dan Fante’s Corksucker back in 2005" -- Mark Raison.
One of Agatha Christie's early novels, The Murder of Roger Ackroyd.
Latest story in December 2013 issue of EQMM.
Eeyore was saying to himself, “This writing business. Pencils and what-not. Over-rated if you ask me. Silly stuff. Nothing in it.” A.A. Milne
Dr Jekyll and Mr Hyde is the classic. Wikipedia has a big list as well.
Be aware that most popular portrayals of Dissociative Identity Disorder are inaccurate to the actual medical condition, though.
Well, I've read a lot of fantasy, horror, and sci-fi novels where someone is possessed/controlled by an outside force at times. Does that count?