I've also tried to find good books on this - but haven't really found one. I have got three books on NH but wouldn't highly recommend any of them:
Pat Parelli's book - lots of talk - not much doing. I think it's all saved up for the courses and stuff you get later if you sign up for his programme.
Bill Dorrance - True Horsemanship Through Feel. I understand that this is one of the classics of natural horsemanship and some people just love this book. However, it's written as if it was him talking and this makes it a difficult read. I didn't get through it.
John Lyons - Lyons on Horses - probably the best of the three but still not great.
I've got books on other aspects of horsemanship which are much better. I think I like a lot of photos and diagrams so that I can see what they're getting at. Will be very interested to see if anyone else has some recommendations.