Does anyone have any idea what this book is? My correspondent says it isn't a Jill book. This is what she can remember:
This book is written in the first person: and opens with the girl almost ready to give up riding after a bad lesson. She and her friend are riding along on their bikes discussing the lesson. Once she's home, she finds a letter from her aunt inviting her and her friend to come and look after the aunt's riding school. The two girls go and run a very successful camp for the riding school pupils. The book ends with the girls being invited to come and run the riding school in their school holidays.
The person who asked thought the book was the first in a series.