Mary Louise and the Liberty Girls, by L. Frank Baum writing as Edith Van Dyne

The Mary Louise series is about a girl amateur detective and were written for teenage girls. Set in World War I, Mary Louise and the Liberty Girls explores the themes of patriotism and dissent in wartime. Mary Louise is 17 and teams up with her friends aged 14 to 18 to form the Liberty Girls to help out the war effort. They start with helping to sell bonds, but, when anonymous letters opposing the war appear, the girls go searching for the treasonous pro-German people of the town. Are they traitors or are they just voicing their right to disagree?

Being in the Public Domain you can find a copy of Mary Louise and the Liberty Girls in Project Gutenberg.