“Schooling for Young Riders” by John Richard Young.
He decided to write a book about this. He bought an almost 2 year old stud colt (Arab/Welsh?) with absolutely no training out of the field.
His adventures breaking this colt to civilized behavior and riding under saddle are very helpful. He fully admits to his mistakes! His purpose was to have one of his two daughters learn how to train a young horse under his direction.
This ended up being my main go-to book when I bought weanling Arabian stud colts. I used some of his methods on my already saddle-broke and trained horses. This training book is different, there is no “oh just do this and the horse will do this guaranteed!” idiocy I found in other books.
For after the initial training I depended on “Schooling Your Horse” by Vladimir Littauer which lays out a pretty detailed training method for 9 months with the aim to have a safe horse to ride, jump, hunt and show.
I also consulted “Training Hunters, Jumpers and Hacks” by Harry Chamberlin a lot. This schooling course is longer, 2 years, and is aimed at training horses for cavalry officers who often ended up competing internationally. Very detailed instructions by a guy who was in charge of the Fort Riley cavalry school and who had trained hundreds of horses in his career.
With this really solid foundation you can go wherever you want to go with your horse later, as long as he remains sound of course.