I recently signed up for a subscription to "Grow Your Own" magazine, and was given the free gift of a copy of "The Allotment Book" by Andi Clevely.
It arrived today, and I have to say it looks excellent, a really good in depth guide to having an allotment. It talks about sheds, paths, soil improvement and much more.
Well written, with beautiful pictures and tons of information.
Recommended. Either get it with the subscription, if it's still available, or buy it.