After reading so may of Sanderson’s cosmere novels, it’s a bit odd to read one that isn’t. There’s still a ‘Sanderson feel’ to it, both in how the characters think and interact and also in this novel’s version of a magic system.
Basic premise: the main character–Stephen Leeds–has manifest many ‘aspects’ that only he can see. Each is a specialist in a specific field that Leeds needed to solve a particular problem. It’s left vague if the aspects are actually real (in the sense of being conscious entities, separate from Leeds himself), which makes the story all the more interesting. So far as Leeds is concerned, they are, and his mind will jump through some pretty crazy hoops to keep that illusion going.