The American Black Chamber (there's some controversy surrounding the author, too)

https://en.wikipedia.org/wiki/The_American_Black_Chamber

Enigma: The Battle for the Code (I think that's the one I read)

http://www.amazon.com/Enigma-Battle-Code-Hugh-Sebag-Montefio...

I know I read one specifically about breaking Purple, but can't seem to track it down (I don't believe it's the William F. Friedman bio).

I haven't read this one, but it also seems to be what you're looking for (and has a decent rating):

http://www.amazon.com/Code-Book-Secret-History-Code-breaking...

(You might check a public/university library for these and related titles--that's where I found them.)