Fermat's Last Theorem: The Story of a Riddle That Confounded the World's Greatest Minds for 358 Years

It tells how one person (Andrew Wiles) worked to prove a theorem that had stumped mathemeticians for over 3 centuries after reading about it when he was in primary school.
Singh explains the ideas behind the eventual solution in a clear manner. A lot of the actual maths baffled me a bit, but they're not the main point of this book.
Rather, it shows the single minded obsession that someone can have as they try and solve something that possibly only a few others could.
The book is well written and has an interesting story behind it, with tension rising in the last few chapters. A good mix of ancient and modern mathematics history, plus interviews with colleagues and peers.