The girl-detective concept had a persistent hold on Baum's imagination. He returned to it in a more successful series, the Mary Louise novels that he began in 1916.
It is clear that Baum had hopes of more Daring Twins novels, involving the younger of the five Daring siblings and eventually their children as well. Evidence suggests that he wrote at least a third book in the series; in the papers left after Baum's death in 1919, the file that contained the manuscript for his last Oz book, Glinda of Oz, was labelled Phoebe Daring, Conspirator. Baum's correspondence with his publisher, Reilly & Britton, mentions yet another book, titled either Phil Daring's Experiment or The Daring Twins' Experiment. Yet nothing of these other Daring books is known to have survived.
Like The Daring Twins, Phoebe Daring involves two orphaned twins, Philip and Phoebe Daring; as its title indicates, the sister takes the primary role in the second book, which delivers a plot about a good man unjustly suspected of a crime – very much as the first one did. This similarity, and lack of originality, might be the best explanation for the book's limited popular success and the termination of the Daring Twins series after two books.
