Fans of mathematical fiction are likely to love the selfreferential nature of this novel about a timemachine repairman whose future self travels back in time to give him a novel about a timemachine repairman whose future self travels back in time to give him a novel about....
However, even though such selfreferentiality is of mathematical interest, this essential aspect of the book does not qualify the book for inclusion on this website according to the "axioms" I have set forth. According to these rules, only explicit mathematical content qualifies, and here the explicit mathematical references are essentially only "window dressing". For example, consider the relatively unimportant references to foundations of set theory in this key quotation from the book:
(quoted from How to Live Safely in a Science Fictional Universe)
Normally, when someone says trust me, I find it hard to trust him anymore, and this is doubly true for when it is my self who is saying it, but as it turns out, in my science fictional studies, I once took a course on topological properties of possibility space and in chapter three of the coursebook we had covered this very scenario as a case study in this:
Exceedingly Improbably yet Hypothetically Still Possible States of Affairs in a Coherent Universe Governed by a Consistent Set of Fictional Laws
and in fact, for a while I even considered writing my thesis on a minor but novel approach to proving, with only ZF+CH (ZermeloFrankel set theory plus the Continuum Hypothesis), that this exact fact pattern, the one happening to me right that moment, was in fact (i) grammatically allowed, (ii) logically permitted and (iii) metaphysically possible. And of course, my future self would know all of this, and he would know that I would know that he would know this, and that's why he knew it would be worth it to give me this book. And so he's written in his handwriting, handwriting I recognize as my very own, these words:
Read this book. Then write it. Your life depends on it.

Or the nonEuclidean geometry in
(quoted from How to Live Safely in a Science Fictional Universe)
If, in connection with a repair job, I have to do some backoftheenvelope physics, nothing fancy, just some roughandready number crunching, there's a spacetime simulation engine with a touchscreen interface that offers dropdown menus with easytouse partial differential equations; all I need to do is click on what kind of geometry the universe has in the local region (Euclid/Riemann/Lobachevsky) and I'm off to the races.

All of these mathematical references show that Yu really knows his mathematics, but they are not so significant to the plot that the reader needs to know anything about them. So, I will be giving this book a relatively low rating for "mathematical content".
This does not mean that I am at all criticizing the book. In fact, I found it fun and beautiful. It is, I think, what Richard Brautigan would have written had he been a nerd instead of a hippie. (And from me, that is a real compliment.) If read literally, it makes no sense. The main character, whose name also happens to be Charles Yu, lives in a science fictional universe where people study laws of fiction instead of laws of physics. His cousin works on the Death Star and tells him what a great job it is. Academics in Minor Universe 31, where he lives, debate whether "reality" is a special kind of science fiction or whether it is something completely different. On the other hand, in a mysteriously poetic way, the nonsense says a lot about our real lives, about how we deal with our parents (a major theme of the book), about destiny and success.
This sort of writing may appear too avant garde for some readers, but if you can handle a little weirdness and would not be offended to be called a "nerd", this book is highly recommended reading for you. 