A book from an author I thoroughly enjoy, Simon Singh, "The Code Book" is extremely entertaining and educational. One of the latter sections of the book discusses the development of public key cryptography. It is one of those books which tells the backstory, describing the events and the people involved. It's a wonderful book, and while there isn't an e-book version, there is a
CD version of the book. One of the very interesting features of the CD is a working virtual Enigma, a WWII-era German encryption device.