Set This House in Order by Matt Ruff

Andy Gage was born in 1965 and murdered not long after by his stepfather. . . . It was no ordinary murder. Though the torture and abuse that killed him were real, Andy Gage's death wasn't. Only his soul actually died, and when it died, it broke in pieces. Then the pieces became souls in their own right, coinheritors of Andy Gage's life. . . .

While Andy deals with the outside world, more than a hundred other souls share an imaginary house inside Andy's head, struggling to maintain an orderly coexistence: Aaron, the father figure; Adam, the mischievous teenager; Jake, the frightened little boy; Aunt Sam, the artist; Seferis, the defender; and Gideon, who wants to get rid of Andy and the others and run things on his own.

Andy's new coworker, Penny Driver, is also a multiple personality, a fact that Penny is only partially aware of. When several of Penny's other souls ask Andy for help, Andy reluctantly agrees, setting in motion a chain of events that threatens to destroy the stability of the house. Now Andy and Penny must work together to uncover a terrible secret that Andy has been keeping . . . from himself.