The now-defunct History Mysteries were each one-off stories, set in different times and places in American history. They're a little longer and aimed a little higher than the main American Girl books. The Night Flyers is set in World War I era North Carolina. This particular plucky girl detective sets out to discover who's stealing her birds and if the new man in town with a strange accent is a German spy. There's actually quite a bit going on here, but it all ties together fairly well in the end.