I've just finished reading
In Code: A mathematical Journey; a very interesting book about cryptography, written by a 16 year old girl who invented a new algorithm. I didn't know very much at all about cryptography before reading it, but now I've got much more of a feel for it. I could just about follow what was going on in the book, but not enough to really understand it all. It was a great read though, and I so recommend it to you.