I use Clinton Anderson which is very easy and understandable and easy
For the horse
I love it,it has brought my horse so far on ground manners and under saddle problems
Want to mention that natural horsemanship and positive reinforcement don't mean never using a reprimand or setting boundaries with a horse (or any animal). There are a lot of nuances to it. If I have a horse that wants to drag me into the feed room every time we walk by, there are several positive reinforcement techniques I can use to modify that behavior, but does it mean I am NEVER going to give a sharp tug on the lead line? Depends on the decision the horse is making and how severe the behavior is. You have to be ready to reward any step in the right direction by the horse no matter how frustrated you might be., but setting no boundaries isn't good either. Clinton Anderson has a video helping an owner with a severely aggressive horse. Here is the link:
https://youtu.be/CjjRuBNiJ1k They aren't using force or aggression against the horse but they are putting pressure on to mitigate bad behavior. Two good reads that can help are Zen Mind Zen Horse and for basic operant conditioning, read Don't Shoot the Dog by Karen Pryor.
I don't have any experience with it, but I've heard it's really good! I encourage you to check out thewillingequine.com, a website that talks about positive reinforcement and the benefits of it.