I have read, and watched alot of Clinton's methods, and they aren't any more 'aggressive' than alot of other NH trainers; he starts out soft, and puts more pressure on as the horse delays response.
To the OP, I don't remember the author, but "Natural Horsemanship Explained" is an awesome place to start, as it doesn't endorse any particular trainer, but gives alot of detail as far as what a natural horseman is looking for, and trying to achieve from his horses. I read it at a bookstore one time, and if I had had the money, I would have bought it! Alot of people on here will tell you go with a certain trainer, but I think you need to gain the actual incite of what NH is first...THEN you can choose a trainer you like from what you already know. Personally, I glean incite from ALOT of trainers, not just one, and I don't usually recommend one over another, because horses are going to respond to training methods differently, and a huge part of horse training, is learning what your horse can handle, and how you can best influence him to do what you want without fighting him.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."