For most of the book, the fact that the protagonist studied mathematics is essentially irrelevant, aside from perhaps giving an overall impression that she is capable of thinking logically. At one point, she is told that she should be proud of having a maths degree from Cambridge (even though she rated only third class). The part about the Monty Hall Problem, though not especially significant to the overall plot, is many pages long and somewhat interesting. She is trying to help her boyfriend to incorporate the problem into a story called "Probable Adultery" that he is writing about a man who knows that his wife and her lover are behind one of three doors in a hotel.

Thanks to Tom Louis Lindstrøm for letting me know about this interesting and well-written (though only barely mathematical) novel.