A book that I enjoyed and found helpful which wasn't on the reading list was Prime Obsession: The History of the Greatest Unsolved Problem in Mathematics by John Derbyshire. This is a book about the Reimann Hypothesis which alternates a history of it with a mathematical explanation of why mathematicians care about it and its relationship to prime numbers. It's a good read and accessible with AS (I think) level maths but reaches a long way with that maths. It also provides a number of little tidbits of maths if you're asked to say something you've studied in an interview, such as proving ∑(1/ns)=π(1-p-s)-1 (this is the Euler Product formula, where n are the natural numbers and p are the primes, and is the first step to deeper results about the Reimann Zeta Function (which is what the Reimann Hypothesis is about) and the primes.
Ben