Enid Michael has written just this one children’s book, The Runaway National. After a slightly unbelievable start, when the teenage Kerry manages to outwit a particularly nasty gang of villains (and I admit my heart was in my mouth when Kerry was taken off to stay with them: the book was written in more innocent times, thankfully), the book settles down and turns into a heartwarming tale of a horse in whom nobody has believed apart from Kerry.

Finding the book
Very easy to find, and usually cheap.

Sources and links
The British Library catalogue