Trigonometric Delights by Eli Maor
Gamma - Exploring Euler's Constant by Julian Havil
The Story of sqrt(-1) An Imaginary Tale by Paul Nahin
Thinking Mathematically by John Mason with Leone Burton and Kaye Stacey
Proofs and Refutations - The Logic of Mathematical Discovery by Imre Lakatos
Visual Complex Analysis by Tristan Needham