First of all, I have to say that I'm a big fan of the Cool Camping England and Wales books - I've used both of those and they have directed me to some camp sites that really are excellent :- ones which are off the beaten track, lovely locations, not over-commercialised, as close to wild camping as you can get without actually wild camping!
However this Cool Camping book for Europe is, i feel, far less researched (and i'd have a similar issue with the one for France which i've also used). Having used both, my impression (and i may be wrong), is that for Cool Camping England and Wales, there really was an element of looking for campsites that were not mapped /covered in other books, whereas for Cool Camping Europe (and Cool camping France), well, for the entries covering France and Spain at any rate, i feel they just did what I did: looked for "camping" signs on the Michelin map and just went to those. I don't even feel much effort was made to ensure that the campsites they were recommending were the best in the area or not. For example, when travelling in Spain, we found our Michelin map (which had little "caravan" signs to denote campsites), covered all the ones listed within the book, and many more besides, and, after a few cool camping disappointments, we found ourselves checking out all the marked campsites in the area, not just the cool camping ones, and frequently, we chose the one not mentioned in the cool camping book.
I myself would be wary of anyone who gives this book a high rating but who hasn't actually used the book yet - yes, the book is beautifully presented, with some stunning images and yes, it is well written, and yes, looking at it can get you very excited about visiting the various locations, but .. when you actually get there, it's another story.
Note - they actually dismiss Belgium in a few sentences, but when we went there, we found several fairly decent campsites - as decent as the ones the book had recommended in France at any rate (for example, along the valley of the Semois river if you are interested!) - yes, they had static caravans, but again, so did many of the sites recommended by Cool Camping Europe in France and Spain.
So to summarise - Cool Camping Europe is as beautifully written and presented as the others in the series, but not as well researched as their initial offerings. Use it in planning your trip, but make sure to check out other independent reviews, and to check out the camp sites own websites. Also, if you're planning any sort of longish road trip, you'll need another source, since the campsites covered can be few and far between.
I've spend quite a few months camping in France, Belgium and Spain this year so my comments are restricted to those areas.