I also recommend “Horse Brain, Human Brain” by Janet Jones. I think that every person who interacts with horses NEEDS to read this book.
The most re-read book in my library of horse books is “Common Sense Horsemanship” by Vladimir Littauer. I reread it every three years or so to make sure that I am on track with my riding. Whenever I am in doubt about anything about my riding/training this is the first book I check for ideas.
“The Way to Perfect Horsemanship” by Udo Burger helped me transform my riding to the next level of riding WITH the horse, especially for effective and gentle aids that work well and do not irritate the horse.
If I had to shrink my equestrian library down to three books these are the ones I would pick without any hesitation.
Another book that could answer many questions is “Simplify Your Riding” by Wendy Murdoch. Part Three–“Timing” is quite good.