The Nightcaps books are primarily short story collections, linked loosely by the introductory narrative in several of the books, and occasionally by the narrator's references to stories or events from other books. (At one point in Fairy Nightcaps, Aunt Fanny devotes an entire paragraph to commenting on "that ridiculous picture of our 'Nightcap children' in 'Baby nightcaps.'")
The series premise is that Charley's mother tells him stories ("nightcaps"), which are being retold here. Charley and his siblings, along with several other characters, appear briefly in several volumes and play a larger role in the last book.
Some listings erroneously attribute titles in this series to Frances Dana Gage, who also used the pseudonym "Aunt Fanny." The Preface to the first volume, which refers specifically to requests for stories like Nightcaps (whose copyright links them directly to Barrow), indicates that the Pet books were Barrow's work, not Gage's.
"Pet" here is an affectionate name for children. The Preface explains that this book is a response to parents' requests for stories that children could read for themselves -- thus the limited vocabulary. These are all short story collections.
"Shoot folly as it flies" appears on the title page of each volume. Like the Nightcaps series, Pop-Guns are linked primarily by scenes either before or after the main story, when Aunt Fanny visits a group of children.
Even though Barrow's name appears on the copyright page, titles in this series are occasionally incorrectly attributed to Frances Dana Gage (who also used the pseudonym "Aunt Fanny").
Volumes in this series were co-authored by Fanny Barrow and her daughter Sarah, with the authors listed as "Mrs. S. B. Holly [Sarah's married name] and Aunt Fanny." Apparently written after Sarah's conversion, Twelve Little Sisters is one of the few nineteenth-century girls series highlighting the Roman Catholic faith. The publisher, D. & J. Sadlier, specialized in Catholic literature.