Actually, I just finished reading this book last night. I agree with most of Fusun's comments, but actually the connection between the mystery and the calculus is almost non-existent. There is a lot of discussion of mathematics (his research on bounded operators on Hilbert space, Fermat's Last Theorem, Godel's Theorem, etc.) which is completely unrelated to the case. At only one point in the story, he uses some really elementary computation (the solution to the differential equation dx/dt=cx) to determine something about the case (the absorption rate of arsenic into the body) which he later verifies by asking a toxicologist anyway.

One strange thing for me about reading this book is that it wasn't sufficiently different from my own life that I could really enjoy reading it. There is a lot about trying to get a job as a professor of mathematics, a lot about dealing with calculus students during office hours and some instances of people reacting poorly when they hear that he is a mathematician ("Yuchhh...I hate math.") I get enough of those things in my own life!
(See also "Advanced Calculus of Murder" by the same author.)