What is a good book that has a main character suffering from some sort of mental illness?

Question

What is a good book that has a main character suffering from some sort of mental illness?
I have to read a book and write a summary type thing about it for my psychology class, but I do not know of any good books that have a mental illness as a part of it for the main character. If anyone can help that would be GREAT!!!!

Answer

Sybil is an excellent story about a woman with multiple personality disorder. It would be great for a school project. Its an easy read, its not super long, and its very interesting.