That's a tough one... here are a few suggestions:
Misery or Gerald's Game, also by Stephen King
I Know This Much is True by Wally Lamb is much more of a touchy-feely book, but it is really good. It is a story about twin brothers, one of whom is schizophrenic. It details his descent into isolation and madness very well, and he nearly drives the sane brother insane in the process.
Wicked, the Life and Times of the Wicked Witch of the West also explores someone going mad due to isolation, at the end of the book.
Also- have you seen Insomnia, the movie with Al Pacino? Pacino goes to Alaska and basically goes insane from sleep deprivation. I didn't like it that much, but it does tie in nicely with your theme. I also heard it was based on a much better foreign-language film, so that may be worth checking out.