I just realised that I left out "A Mathematician's Apology", by G.H Hardy, written just before he died and after he had stopped producing original mathematical work, he defends his choice of work and explains his motivations as a mathematician. A short little book (the version I have has a foreward by C.P. Snow that's about as long as the book itself, it's 50 pages long ... )

Just finished reading "The Music of the Primes: Why an unsolved problem in mathematics matters" by Marcus du Sautoy. Obviously it covers similar things to the book I mentioned above by Sabbagh, but I found it more engrossing. He uses the analogies of musical notes of the primes and the metaphorical instruments which create them frequently as he gives a historical account of the various mathematicians who have made major contributions to the search, tries to explain those contributions, and puts them into the context of how mathematics in general was evolving as time went on. I think du Sautoy is probably the most infectiously enthusiastic writer of science matters I've come across since Feynman. I immediately signed up for a course in complex analysis