This particular treatment for MPD is real, though controversial. Readers familiar with the well-known story of Sybil will recall that in the case of that book's title character, the disease was treated by integrating the personalities into one functional consciousness.
While Andrew makes for a compelling narrator, the book doesn't really take off until Penny, one of his co-workers, is introduced. Penny, like Andy, suffers from MPD. Unlike Andy, she is completely unaware of her condition. As such, she experiences lost time, finding herself mysteriously shuttled from place to place after extended blackouts during which her other personalities take over.
That Ruff can manage to make Andy and Penny's various personalities distinct enough that the reader can keep track of them along with half-a-dozen other supporting characters is a testament to his skill as a writer.
An intense yet enjoyable read, Set This House in Order contains at least one shocking twist, though it is played so realistically and is so well integrated into the story that suspension of disbelief is not required. With his third novel, Matt Ruff proves his mettle as a serious writer and a deft storyteller.