This book teeters on the edge of being too clever and snarky, and then suddenly on the edge of being too earnest and depressing, and somehow manages to not fall, which makes it very, very good.
I had more, but GoodReads ate it and produced nothing. Generally, this is a suprisingly smart and fun book that reads far faster than it should, that draws in “genre” and “literary” readers alike with specific terminology and themes, and is especially enjoyable if, like me, one really can appreciate both. It starts out about time travel and different universes, and ends up being almost entirely about a cycle of depression and failure, and it works.
Time travel -- and even more specifically, being caught in a time loop -- is one of those great ideas that one can learn from a cartoon as a child and pretty much entirely understand, but which can become infinitely simple or infinitely complicated depending on how an author wants to present it. Here, it's mechanics can be complex, but that's just fine because it's in service of the character. There are a few points wherein Yu’s concepts and rules become too complicated and cross over from “fun to figure out” or “goofily over-complicated” (there are shades of Douglas Adams when you least expect it) into “what...?” but overall the whole thing is very solid. I am trying to think of more flaws with the book, and I know that they are there, but they do not come to me at present -- so five stars it is, because that's how I felt at the moment I finished it.
A mostly enjoyable, and at times (but not ultimately!) soul-crushing, book that I think I could get a lot out of reading a second time someday. I have no idea what sort of direction Yu will go as a writer from here.