I enjoy math books like:

- e; The story of a number (Eli Moar) - An imaginary tale; The story of -1 (Paul Nahin) - The Poincare Conjecture (Daniel O'Shea) - The man who loved only numbers (Paul Hoffman) - Prime Obsession (John Derbyshire)

They're part biography, part history, and give a little colour to the subject that isn't available from your typical college textbook.