Don't be too put off because of August. There are enough sites along the coast so that, even if you don't book ahead (and by the way, I never do, either) you can still be sure of finding a vacant pitch somewhere without driving too far so long as you don't wait until late in the evening until starting your search. My experience is that so long as I start looking around before around 5pm, I'm nearly always lucky first time even if it's not always the pitch in the first row with an unobstructed view to the beach.
Yes, it is a shame that the ADAC guide doesn't have very many links to the websites of the individual campsites (where they have one) but that is really the only shortcoming of their online campsite guide. Otherwise the information is pretty comprehensive and, above all, reliable. If necessary, Google has usually helped me whenever I really would like to know more.