Today's pony book is Monica Dickens' Word's End in Winter, part of her four book World's End series. The series was one of my favourites as a child. I loved the World's End children, their animals, and their parentless existence. I knew these books just about off by heart. Here's the version I had, and in fact still have, though I will admit to having acquired a hardback version as well.
The World's End series wasn't received well at the hand of critics: Nicholas Tucker said that the books appealed to "a host of pre-adolescent fantasies and prejudices," which of course it did, but as one of those it appealed to, I didn't care. It was a spiky, difficult, but magical world.
You can read more about Monica Dickens here, and wallow in the rest of her pony books, which include the immensely popular Follyfoot series. If you're a fan of the TV version, you can't do better than look at the Follyfoot Tribute Site.
Read more about the pony book in Heroines on Horseback, my survey of the world of the pony book from the 1930s onwards. You can buy a signed copy from me (I'll post them worldwide). The book's also available from the usual sources, or your local bookshop.