Valis by Philip K. Dick, 1981. Working on a series of PKD titles as a personal project: It began with a blinding pink light giving a schizophrenic man named Horselover Fat visions of an alternate Earth where the Roman Empire still reigns. He must decide whether he is crazy or whether a godlike entity is showing him the true nature of the world. Part science-fiction, part theological detective story—God plays both the missing person and the perpetrator of the ultimate crime.
