Book 9 of the Adventure Stories for Girls Series.
Florence and Betty travel to France with their friend Madam Strosser. While visiting the forests and small villages in a simple wagon, they stumble upon an American soldier who had been wounded in the war ten years ago, and who was looking for a dugout that held a treasure. The treasure had been stolen by the Germans from a small town for whom it was their only source of tourist income.
While on this quest, they also come across several gypsies and a young blond-haired lady who seems to be mysteriously connected with them.
Travel with these young ladies as they see many of the wonders of France, and use their courage and creativity to succeed. (Summary by Keith Salis)

I know that the word gypsy is considered a slur by many today, and the term Romani is preferred, but do remember this book was published in 1929. I have not read it so I can’t tell you if it perpetuates negative stereotypes of Romani people.