Purity belongs to that class of story based entirely on its protagonist learning who his or her parents are. It tells us many awful stories in skilled mind-twisting order, with a high-impact revelation of parenthood. It is rich and fat, and bold.
There is something else though. There is more than dash of insanity in several characters, and their motivations are hard to follow. But the text is so tight that I'm convinced that difficulty is just me not putting in enough effort. It's so tight that I'm willing to forgive it anything.