Whatever book you buy most of them will be filled with terms used in equitation - most of which are words for which you think you know the meaning but for which probably you don't.
Jean St Fort Paillard - a Frenchman, wrote a book entitled
In it JSFB explains words like impulsion, straightness, calmness, tension, engagement etc
The book Isbn 0-173-00059-4 is out of print - but try the second hand shops
If you want to learn about equitation you must first understand the language
This one of those books which you will refer back to, time after time.