West Harrow Allotment and Garden Association
There is a vast amount of advice and information on the web:
If you want to know what you should be doing this month then try
Thompson and Morgan Monthly advice
Gardeners World weekly advice
Allotment and Gardens website
The National Allotment Society has advice on when to do what and how but also has information about allotments in general e.g. what size they should be, how to manage an allotment site, etc.