Mary Louise and the Liberty Girls

The Bluebird Books is a series of novels popular with teenage girls in the 1910s and 1920s. The title character is Mary Louise Burrows. In the first books of the series, she is a fifteen-year-old girl with unusual maturity (though the other girls in her boarding school find her somewhat priggish). She is suddenly confronted with the fact that her beloved grandfather is suspected of no less a crime than treason against the United States.