I’m currently enjoying "Fermat’s Last Theorem" by Simon Singh. He has used the thread of the solving of Fermat’s last theorem to tell the history of mathematics. The maths in the main text is kept as simple as possible, so that you can follow the story even if you don’t understand the maths. More complete examples of the maths are given in the appendices.
The American edition is called "Fermat’s Enigma: The Epic Quest to Solve the World’s Greatest Mathematical Problem".