The Cave in the Cliff by Kathleen Mackenzie
 
This is an exciting story about two teenage sisters who go to stay with an aunt in France during the summer holidays.
Kathleen Mackenzie has left the world of horse shows and riding for the time being and this is an adventure story of a different kind. The excitement starts almost at once, while the two girls are on the Channel crossing, and it quickly snowballs into a really big mystery.
Smuggling, burglaries and kidnapping are all vividly described in what is perhaps the best of this author's books.
 
Illustrations by Violet Morgan