There are several European camping guide books - you just need to look in the travel section of your local bookshop. I have an oldish AA one which is perfectly adequate. Also, many general travel guidebooks such as the Let's Go series have campsite details. If you have proper maps (as opposed to an atlas) for the region you're visiting, you often find campsite locations marked, too. Which source of information you use really depends on how particular you are about finding out about prices and facilities before you get there.
Whilst it's true to say that you can turn up on spec at the majority of sites, you need to know where they are in the first place to do that, and signposting can be variable. If you don't take a guide/map, you can always get information at local tourist offices, including in advance via email if you leave enough time for this.