Even though at having been 45 years old, am now a 62y/o great-grandmother, when I got my own first horse I did have many years of experience with horses. My first actual experience with showing at halter showmanship was with Shetland Ponies at local Fairs. Those horses owned by friends and my hiring myself out as a handler and barn help served me well in getting to handle many different horse personalities in my younger years.
Even though actual horse handling is a good learning experience I thoroughly believe in "book reading and learning" along with the physical handling of the horse. One reference book I found by happenstance is "Basic Horse Care" by Eleanor F. Prince and Gaydell M. Collier. I do have a good many books in my personal library on all aspects of horse handling and training.