"Options, Futures and Other Derivatives", Hull and/or "Financial Calculus: An Introduction to Derivative Pricing", Baxter & Rennie.
Either of these books individually would represent a good grounding in the mathematics underlying derivative pricing. The two books are very different to each other, though, and it is worth the reader considering his preferred approach before parting with cash. The main differences between the books are:
1. Baxter & Rennie follow a "pure maths" approach, basing the theory around a succession of mathematical theorems. Hull describes this approach in a later chapter, but builds up the theory using an "applied maths" approach, deriving a partial differential equation satisfied by derivative prices.
2. Hull includes background information on the derivative markets; Baxter & Rennie do not.
3. Hull describes how derivatives can be priced in practice, using techniques like Monte Carlo and trees; Baxter & Rennie do not.
If I had to choose one book, my personal preference would be for Hull, but this probably reflects my choice of degree courses. But having read Baxter & Rennie after Hull, my opinion is that the books compliment each other well. When things get so complicated that the intuitive realism of applied maths needs to give way to abstract pure maths (for example in considering quantos or yield curve models), the Baxter & Rennie approach is easier to follow.