It is a British-Russian spy novel in the style of Le Carre that is set in Cambridge, UK. If you like that sort of thing, fine. It is true that the murdered genius is a math graduate student, and he leaves behind a number of files about famous unsolved cases, to which he supposedly applied principles of mathematics. In fact, at the end, his solution to the Kennedy assassination (yes, it was the Cubans) is to be published posthumously. There are no specific details of his solution method (except vague use of geometry and otherwise standard detective work), and frankly, the recent Peter Jennings program where the "mathematical model" that kinda proved that everything was exactly as it seemed, and that the one shot was fired from the book depository, impressed me as much more realistic (I don't know if the model was checked out by reputable mathematicians). "The Kennedy Theorem" above is the only place where mathematics plays a role. "The Cambridge Theorem" in the title is the deceased's solution to an unrelated British spy mystery, namely, the identity of the fifth Russian mole at Cambridge, whose four colleagues were unmasked a long time ago. Towards the end, the plot deepens and gets improbably complicated, to be revealed in a torrent of brilliant intuitions on the detective's part. You can tell I was not overly impressed by the literary style either.
Overall: an interesting book to read, if you like the genre, but it is not about mathematics or mathematicians (even the dead guy's advisor is a physicist). And I wouldn't bother to read it twice.