The book you want is this one;
"Federation Francais de camping et de caravanning"
It gives all of the camping sites in France, even the small ones ones which are cheap but perfect for the traveler. It is cheaper to buy in France but as it is a substantial book it's not recommended that you carry it on tour.
That said, you shouldn't need it as along the valley of the Loire there are hundreds of camp sites and in June there should be no need to book. This is a popular route for cyclists in that it is gentle country for cyclists and is therefore well served by camp sites.
I shall be taking it myself sometime in July in order to get across to Roanne and then north up through Burgundy.
Good luck with the tour but be aware that this is forecast to be the hottest year on record.