This is the "Bible" for those who are wanting to camp in France, and is constantly updated, incorporating user comments. A single multilingual version is produced every year, although it does help if you have a smattering of French (which campsites tend to appreciate anyway!). It comes with a wealth of helpful regional, local area and town/city maps, which give an indication of where each campsite is located.
For a fun fortnight in France, book the first couple of nights only, and then see where you fancy going after that. Just make sure you take the book with you, although you should manage to pick up another one in France if you forget... (there's a great bookshop in Chartres that we visited for that very reason!)
We really enjoyed some of the campsites on Ile de Re ("La Providence" in Les Portes-en-Re) and in Larmor Plage ("Camping des Algues") but see what takes your fancy. The 'municipal' sites are usually pretty good value for money, although occasionally they can be rather grim and basic compared to the better independently-run campsites.
Bonnes vacances!