Depends entirely your mode of camping:
If looking for a nice leisurely stopover then both main clubs, Caravan and Motorhome and camping and caravanning, offer a wealth of variety and choice, from full facility superb if regimented club sites to small 5 site max offering from nothing but water and a waste dump to full facility.
Both have books and websites for choosing with.
One-nighters not bothered about full facilities? Britstops lists over 700 pubs, farm shops etc where owners allow you too stop over night hopefully in exchange for business?
Wildcamping? Lots of websites.
A composite website? searchforsites is excellent.
Overseas? All the Aires from vicarious is excellent, a guide to around 9,000 places to stop the night for free or a few Euro's.
So the list goes on ....
UKcampsites is a good site also.
ACSI for out of season, they have paper guide books as well as an App,,,