dissociative identity disorder.
The best non-fiction account on the subject that I've read is "When Rabbit Howls," by Truddi Chase and the Troops. The best depiction of the disorder in a comic book has to be in Grant Morrison's "Doom Patrol" (whose super-heroine with MPD, Crazy Jane, really kindled my interest in the topic).
The best novel I've read about MPD/DID has to be "Set This House in Order: A Romance of Souls," published by Matt Ruff in 2003. It's a marvelously constructed book: smart, heartfelt and full of surprises.
"Born" only two years before the story begins, Andy Gage is the alternate personality that deals with the outside world, with more than 100 other souls hidden away in the imaginary house he's constructed within his head. Through his workplace, he meets Penny Driver, another multiple, one who doesn't quite suspect that she has any alternate personalities. When some of her souls ask Andy for help, he finds himself vulnerable to his own long-suppressed secrets, ones that threaten to destroy the safe interior landscape he's built for himself.
It's hard to write about something as complicated and as emotionally fraught as DID, which usually has its origins in horrific childhood abuse. But Ruff manages to suggest the impact of Andy's and Penny's backstories without letting them become grotesque and exploitative when finally revealed. The novel's subtitle marks, I think, an important distinction. "Set This House in Order" is ultimately a hopeful book, one that overrides the seeming outlandishness of its premise and reveals something true about identity and love.
Ruff, by the way, is also the author of "Bad Monkeys," another twisty tale of identity, which I reviewed favorably in my Chronicle column back in 2007. If you haven't yet discovered him, you're missing out.