Horse Psychology? YOu mean how horses think? Or horsemanship?
I like many of the books written by Mark Rashid. His approach to what he calls "passive leadership" is interesting.
"True Horsemanship Through Feel " is a must read, but is hard to comprehend at times because it is written in the exact words of the speaker, and he speaks kind of "down home cowboyish". I think you are not a native speaker of English, so you might struggle with this.
For enjoyement, have you ever read "Chosen by a Horse"?
I personally LOVE Monty Roberts- The Man Who Listens to Horses and A Horse Called Shy Boy
Theyre kind of centered more on his life but he does have some really good points of view on how the horse thinks, feels, etc