The Jill series of pony books by Ruby Ferguson was one of my favourites when I was growing up in the 70s. Like many pre-teen girls I was horse mad. I managed to nag my parents into buying me a horse by age 10, but I lived in suburban Melbourne, not in the English countryside, so my adventures didn’t really match up with those had by Ruby Ferguson’s Jill.
The Jill series was originally written in the 50s, and I read the books under various covers from the library. My favourites were the green Knight Books series, published in the early 70s. If someone knows who illustrated the covers please share!