This book is certainly more about sex than it is about mathematics. However, I find the one mathematical passage in it so hilarious that I have to include it here.
The premise of the book is that the main character, Arno Strine, is able to temporarily halt time. (The title of the book is the name of the musical notation for a drawn out note, held longer than usual.) Strine does not use his power to steal (when he takes something from a store, he leaves approximately the right amount of money and a note in the cash register to explain) but to use women and their bodies for sexual pleasure. Here he discusses the moral issues:
(quoted from The Fermata)
I would condemn in the strongest terms anyone else who did what I have done. But the thing is, I did it. I did it, and I know myself, I know that I mean no harm, I mean well. I want simply to know what every woman looks like and feels like. I mean only to appreciate what the ribs of a complete stranger feel like under my hands, or to hold some hair I haven't held before, or to come in someone's face while she is paused in her own orgasm.

It is an unusual idea, presented as some hybrid of literature, pornography, humor and science fiction. If you are offended by this passage, though, don't read the book because it gets much more explicit than that!
Strine uses various means to freeze time. For a while he is able to do it merely by clicking a mechanical pencil. But, in the only mathematical passage in the book, the pencil trick doesn't work so well by itself anymore and must be supplemented with "the special equation that [he] had adapted from a journal of mathematics."
(quoted from The Fermata)
"I wrote it on the placemat: the Strine Inequality. I had come across the germ of it in the Birkhoff Library at Harvard on a Sunday afternoon
in a state of Tourette's syndromish meditativeness that I knew by now often presaged a Fermata discovery. I opened an issue of The
Canadian Journal of Geometry at random and was surprised by how many symbolic systems mathematicians had pressed into service:
Greek and Russian letters, of course, but the British pound sterling sign? Capital letters in a florid script that looked as if it came from a
wedding invitation? From a short paper entitled 'Minimally Gilded Hodge Star Operators and QuasiOrdinary Handlebodies Within a
Localizable 4Manifold Whitney Invariance,' I copied out an equation, as follows
[Mathematical notation, including a tensor, the word "End" (for "endomorphism group"), the blackboard bold C and R for the complex and real numbers, but meaning absolutely nothing as far as I can tell.  ak]
Several hours later, at the Ritz Carlton bar, guided by a will greater than my own, I substituted several of the international textile
carelabeling symbols for key variables in the original, and changed the equal sign to a lessthanorequalto sign. I felt as if I were
speaking in tongues as I watched my possessed hand draw a crossedout iron and a crossedout triangle ('no bleach') and a stylized
halffilled washtub with a large hand in it ('hand wash'). When I had finished with the substitutions and the Strine Inequality stood complete
on the page, there came a sound, a sound of distant chronic liposuction, of fine cosmetic work being done on the cosmos, nips and tucks
tactfully taken, infinitesimal hairplugs of time removed from distant starsystems, where they wouldn't be missed, and arranged in quantity
serially for me to live through. I was free once again to roam the Fold. To return to time I only had to erase the inequality sign, disabling its
potency."

(I learned about the mathematical content of this book from Nik Weaver's "Math in Fiction" website. Check it out for his review of this book and some others.) 