Mary Louise Solves a Mystery

The Bluebird Books is a series of novels popular with teenage girls in the 1910s and 1920s. The series was begun by L. Frank Baum using his Edith Van Dyne pseudonym, then continued by at least three others, all using the same pseudonym. Baum wrote the first four books in the series, possibly with help from his son, Harry Neal Baum, on the third.
In this, the third book of the series, Mary Louise and her Grandfather happen upon a mysterious pair of Americans whilst travelling in Italy. Jason Jones is a failed artist, and his companion is his daughter, Alora, an heiress. When the girl is kidnapped, truths stemming back to the time of her parents' marriage are brought to light by Mary Louise and her friend Josie O'Gorman.