Today's pony book is Judith M Berrisford's Jackie and the Missing Showjumper. The Jackie series was written over a 25 year period, and although the girls (scarcely) age, life in the 1980s was very different from 1958 when the first book of the series, Jackie Wins a Pony appeared. This book is shows Jackie and Babs lurching into the 1980s and its world of pop stars. Being utterly pony-mad, the best pop star to Jackie and Babs is one who owns a horse. This pop star owns a show jumper, and Jackie and Babs are going to be spending their Christmas holidays looking after it.
Judith M Berrisford is an interesting author. Judith M Berrisford was a pseudonym used by a husband and wife couple, Clifford and Mary Lewis, Berrisford being Mary's maiden name. Berrisford produced one of the longest pony series with the Jackie books, clocking in at sixteen titles, as well as over fifteen other pony stories.
You can read more about Judith M Berrisford here, as well as see pictures of her pony books.
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.