Augusta Huiell Seaman (1879-1950) graduated from Normal College in New York City in 1900 and went on to teach elementary school. Following her marriage in 1906, she devoted her time to writing books for the children/YA age group, with a focus on mysteries. Between 1910 and 1949, she published 42 books as well as short stories, nonfiction pieces and serialized versions of her novels in popular magazines of the day.
She's considered one of the earliest writers of mystery stories for young girls, and her books remained popular and reprinted even in Scholastic press paperback editions as late as the 1970s. Bookseller Christine M. Volk called her "Nancy Drew for Smart Kids." Unlike the Nancy Drew series, Seaman's books didn't feature recurring characters per se, but typically revolved around two ordinary young girls solving a mystery they happened to stumble on in their hometown, inspired by their intelligence, curiosity and determination.
Also unlike Nancy Drew, who was a globe-trotter, the characters in Seaman's books mostly take place in rural New Jersey locations similar to where the author spent her youth. In fact, a New Jersey newspaper reporter once wrote for the Brick Communicator that he was able to trace the characters' route from The Slipper Point Mystery, matching Seaman's descriptions to area landmarks and buildings. The girls in the stories also reflect Seaman's life in other ways—the girls often have only one surviving parent (in Seaman's case, her father) are often sent to live with other relatives. It's not unusual to find characters who are in poor health (six of the author's eight siblings died young and Seaman's husband died from cancer in 1922).
Although it can be said that the Nancy Drew series is mostly about solving the puzzle du jour, Seaman's books are equally about character development and relationships. As editor Mary Mark Ockerbloom noted about Sally and Doris, "I doubt that Nancy Drew or the Hardy Boys would seriously discuss whether to abandon an inquiry out of concern for its possible impact on another person, as do Doris and Sally. Seaman's characters develop and change."
Although Seaman's books are difficult to track in print, with the exception of a few more recent reprints, there are various free digital versions online, including Google Books and Project Gutenberg.