You may as well ask, 'How long is a piece of string?' There are so many campsite guides out there and I am sure everybody has their favourite. For myself, I like the Alan Rogers Guide. It gives a good (and honest) description of each site, everything from big 5 star sites to small municipals. I have managed to locage several small, quiet sites that suit me fine using this guide.
No doubt you will now be bombarded with suggestions. You will have to pay your money and take your choice.