As I recall, it is mostly medical, growth and nutrition, which is valuable... but as for other books that are mostly training, I've flipped through a few good ones but can't remember the titles.
I like Starting a Colt but it's western discipline. The Foal to Five Years has the training broken down by age and progression, it is English but still all the basics are great.
I never though of Sarah Lee being an overo. If, big if, I breed her I had thought of a local overo that looks like Okie.