Mary Oldham has written just the one pony book published in England: I’m pretty sure The White Pony, published in the USA, is also by her. A Dream of Horses is one of those career-hits-the-horse-dream novels: in the end, the heroine has to decide between an equine career and a librarianship degree. The compromise she comes to, although thoroughly realistic, is probably one reason why the book is not more popular. It is also a good horse rescue story though, and it does have quite a reasonable cover!
Finding the books
A Dream of Horses is reasonably easy to find, and not too expensive. The White Pony is very easy to find in the USA, and copies do turn up in the UK occasionally. It isn’t generally expensive.