The Allotment Specialist (Specialist Series) Easy straightforward guide.
Vegetable Growing Month-by-month: The Down-to-earth Guide That Takes You Through the Vegetable Year Brilliant month by month guide, lots of down to earth (pun intended) useful information.
50 Ways to Kill a Slug (Gardening) Amusing, but also informative
RHS Grow Your Own: Veg (Royal Horticultural Society Grow Your Own) Big book, lots of information
The Essential Allotment Guide: How to Get the Best out of Your Plot John Harrison does write a very readable book, and he is very much an authority on allotment growing