Fermat's Last Theorem

Simon Singh writes more of a history of a mathematical puzzle than about the puzzle itself (though he does explain that too in a clear and understandable way)

Unweaving the Rainbow

Richard Dawkins sets out in this book to arge that understanding the origin and evolution of life doesn't mean you have to wonder at it any less.

In fact he makes a very good argument for wondering at the beauty of it even more.