Graham Farmelo also wrote The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom [0] which I thoroughly enjoyed reading. It's not very technical, which is sometimes a nice attribute in these sorts of biographies, but it covers the science as it develops, if a little too briefly.

[0] http://www.amazon.com/The-Strangest-Man-Hidden-Mystic/dp/046...