Try:
http://bookstore.ucdavis.edu/Display.cfm?itemId=2349
It isn't really cheap, but it has some good info.
The first book on bananas I got was Lessard, W.O.: The Complete Book of Bananas. It is older, and not a lengthy read, but I like having it.
Some other, depending on your needs:
Gowen, S. ed. (1996) Bananas and Plantains. Chapman & Hall, London ISBN 4-412-36870-6
Stover, R.H., Simmonds, N.W. (1987) Bananas 3rd ed. Longman Scientific & Technical, New York. ISBN 0-582-46357-2
Keith