I got an allotment last year, and I've found "The Half Hour Allotment" by Lia Leendertz to be really helpful. It has a guide to what is worth growing and what isn't, and how many of each plant you should bother growing. Mind you, when buying it I did think the title referred to half an hour per week, when in fact it's per day!! But I've got away with only doing an hour per week...