Great question. The "Expert" series by D.G.Hessayon are generally recgnised to be the "bible", so for allotment gardening that means the Fruit Expert and the Vegetable and Herb Expert. They don't specifically deal with organic growing though. If you can find it, Food From Your Garden by the Readers Digest is my number one recommendation. Read my
review to find out the reasons why. Onw I would suggest you avoid is The Allotment Book by A.M. Clevely. It's a nice "coffee table" book with pretty photos, an extensive section about the history of allotments, and a quote from a pop singer on the front, but it's practically information-free. It's OK for inspiration and getting your enthusiasm going, but that's about it.
I don't have a specific organic fruit and vegetable gardening book. Can anyone recommend one?