Book starts with a fairly straightforward premise which I immediately grasped, and it was clear what had happened almost immediately although the protagonist didn't figure it out till much later. But despite what seemed like a predictable start that's been tackled many times in science fiction writing, the inevitable resolution to the protagonist's problem became much more complex and nuanced than any other time I've seen this premise tackled and about half way through the book I couldn't put it down again. If spoilers bother you then I strongly recommend you just read the book without reading any further reviews. But without any spoilers I'll just say the book uses quantum theory to raise some very interesting philosophical questions around regrets for paths not taken, how a person can change based on life choices, and what identity of self really is in the end. I will definitely be thinking about this book for days still.