Books by Tom Dorrance & Ray Hunt. These are the guys who came before Parelli, Lyons, Anderson, Cameron, etc.... In Ray's book Think Harmony With Horses he talks about the very basic idea that encompasses the entire NH movement:
That it's not about making a horse do anything, but about presenting him with an idea and letting him figure it out AND that the training isn't for the horse, but for US (people)....that by working on ourselves (our communication skills through becoming more aware of how we are presenting ourselves to the horse)....that is the important thing. That's what makes the difference.
For example, I can say I am the leader between me and my horse. But if I lead him around by pulling on the lead rope or let him drag me around by the lead, or if he steps in toward me I always step out of his way so as not to get stepped on....then wonder why he bucked me off, or why he bites me or why he.... I'm out of sync with my horse and am only being a leader in my mind not in my actions.
I think the Revolution in Horsemanship is a good book to get you started as to why this NH stuff works, what's the point? why it even "exists" (it's been around for as long as people have existed...practically, but it has just now come around to being so public).....
It's way more than just the mechanics of things....understanding the "why" behind it is (I think) more important than getting the "moves" down perfect.
For DVDs and such:
Clinton Anderson is plain and simple and direct
John Lyons has some DVDs out, and some books that can get you started (Communication through Cues) that explains and has some How to's in em.
Parelli has the games and a lot of stuff too
These, I think are the ones who you might like the best because they break it all up into baby steps for you to put together as lessons.
There's also Dennis Reis who has a complete program like Parelli, Chris Cox has some cool stuff (but I tend to lean toward saying he's more of a trainer's trainer. less ground work and more in the saddle and some folks can't start up there, ya know?)
If you get RFDtv, you can see some Clinton Anderson and Craig Cameron and others...