A must is Tom Dorrance - repeat Tom - entitled "True Unity" - it is difficult to understand but the modern principles of Natural Horsemanship started with this wise, bent, old, cowboy.
And see if you can find a copy of " The schooling of the western horse" by
John Richard Young SbN 85131 182 2
Don't get too hung up on what is the latest fad - nothing is new about horses.
Treating a horse with respect is the fundamental starting point with most of the old horsemasters with perhaps one or two exceptions like the Duke of Newcastle & Baucher.
Remember to know how to do something correctly - you must also recognise how it is done incorrectly.