Stella is an woman with autistic tendencies who falls in love with the gigolo she hires to help her overcome her problems with intimacy. This romance novel, we are told, was inspired by the true life story of its author who began to identify as autistic only as an adult. But, what is it doing here on my list of mathematical fiction?
Let us begin this analysis by breaking the rule and judging this book by its cover. The title itself implies a connection to mathematics. Moreover, the word "Kiss" in the title is typeset under a radical sign. ("The square root of kiss"?) Finally, the background image (behind a kissing couple) is made up of mathematical notation. (I see lots of double integrals, some sort of tiling, the graphs of some functions and part of a trigonometric diagram).
Even though math is not directly related to the plot, it is clear that math is an underlying theme. Stella works as an economist and has developed algorithms for predicting consumer behavior. And at one point she tells her lover
(quoted from The Kiss Quotient)
"I like you better than calculus, and math is the only thing that unites the universe."

So, despite the fact that Stella is arguably not a "professional mathematician", I am going to tag this entry to identify it as a romance with a female mathematician. 