The Pullein-Thompson sisters all rode and wrote pony books. I have yet to read enough of them to figure out differences between the sisters' writing style; I've enjoyed books by all of them.
In Ponies on the Trail, fifteen-year-old Sandy and her younger brother Fergie get a job as assistant guides on a pony camping trek. (Fergie is for Fergus, not the singer or the Duchess of York, whom I spent several years thinking had started a second career as a pop star.) Both ponies and clients are an amusingly mixed bunch, the logistical details of the trek are very believable and a lot of fun, and Sandy especially gets to shoulder new responsibilities.
There's a mystery subplot which is not terribly mysterious, but this is compensated for by a delightful subplot involving a Russian refugee writer who is unfairly suspected of two totally unrelated crimes, and gets the vindication that all writers dream of.
One of the Jane Badger reprints of classic pony books as inexpensive ebooks.

In Ponies on the Trail, fifteen-year-old Sandy and her younger brother Fergie get a job as assistant guides on a pony camping trek. (Fergie is for Fergus, not the singer or the Duchess of York, whom I spent several years thinking had started a second career as a pop star.) Both ponies and clients are an amusingly mixed bunch, the logistical details of the trek are very believable and a lot of fun, and Sandy especially gets to shoulder new responsibilities.
There's a mystery subplot which is not terribly mysterious, but this is compensated for by a delightful subplot involving a Russian refugee writer who is unfairly suspected of two totally unrelated crimes, and gets the vindication that all writers dream of.
One of the Jane Badger reprints of classic pony books as inexpensive ebooks.