It isn't every good series that can handle spin-offs, but Kore Yamazaki's The Ancient Magus' Bride has a particular strength that Jack Flash and the Faerie Case Files takes advantage of: it's rooted in a world's worth of folklore. Because it draws from so many different schools of belief, the base series is able to branch out into other parts of the world, using shared mythologies as a binding agent while still allowing for more in-depth usage of specific cultures' folklore. In the case of this manga, the story takes place in New York City and incorporates aspects of basic fairy lore – changelings – while also bringing in ghost stories and elements of Voodoo mythology, along with some bits and pieces of pop culture.
The story follows Jack (short for Jacqueline, but don't call her that), a faerie woman whose parents exchanged her for a human baby, making both children changelings. Larry, the boy with whom she was exchanged, seems to have been aware of this, but for Jack it's just manifested as a sense of wrongness in her life, a feeling that she doesn't belong. When Larry one day shows up at her window, Jack is at first thrilled, but the delight wears off as she realizes that she's never going to get an explanation or meet her biological parents because the fae operate differently than humans – and despite her faerie blood, Jack has been raised human. It's at that point that she decides that if she wants to do something different with her life, it's up to her to do it, and recalling her childhood heroes Kinsey Millhone and Vic Warshawski, she sets herself up as a private detective, specializing in supernatural cases.
Already we can see the level of research that was done before the story was ever written. Not only is it common in changeling lore for female fae to be exchanged for human males (possibly showing a gender bias that transcends species), but rather than rely on detectives like the staid Miss Marple or other older characters, author Yū Godai (in collaboration with Kore Yamazaki) has picked two much more modern sleuths for Jack to model herself after – Sara Paretsky is still writing about Warshawski (the most recent novel being 2020's Dead Land) and Sue Grafton wrote about Millhone until her death in 2017. Jack, therefore, is the right age to have been able to read most of their works growing up. Also a nice pop culture reference is Jack's detective agency's name – Jack Flash. While she specifically references the 1968 Rolling Stones song in the text, “Jack Flash” is also the name of a Golden Age comic book character who could fly and, even more pertinently, a 1986 Whoopie Goldberg movie set in Manhattan. Goldberg's character isn't a detective – she works in a bank and gets accidentally involved in an espionage caper – but the location and action-based plot feel like more than a coincidence.
The substance of the manga is divided into two parts: a prologue, in which a Japanese tourist is given a cursed video camera by a fae creature (who doesn't seem to mean any harm but rather is genuinely curious), and Jack and Larry's search for a stolen dragon egg, which ties in the main series by having Lindel hire them. Despite not having much to do with the greater part of the volume, the prologue, called “chapter 0,” does a very good job of introducing the world Jack operates in. It's one that's grittier than the main series, and totally lacking in its undercurrent of bittersweetness. Jack and Larry are urban creatures, fully immersed in their world of video games, taxis, and local thugs. Larry's time in the fae realms has made him into something of a werewolf, but he's the street dog version, mangy and tough alongside Jack's 1980s tough P.I. sensibility. Their realm is peopled with ghosts in old city theaters, gangsters and thieves who think dragon eggs are huge opals, and loa - go-betweens of both Haitian and Louisiana Voodoo who operate in the spaces between Bondye and humans. The loa Jack works with are electronic, living within the Internet, making them very much otherworldly creatures of the modern age, more so than anyone in the main series, and grounding the work in the here and now.
In fact, without Lindel's appearance, Jack Flash and the Faerie Case Files wouldn't feel like an Ancient Magus' Bride spin-off at all. It's very much its own story, an homage to the gritty mystery novels and urban fantasies of the 1980s and 90s. As such, while there are elements you'll understand more if you've read the source series, anyone interested in urban fantasy or traditional folklore in an urban setting will very likely be able to pick this up and enjoy it. With its busy, detailed art (which relies on thick black lines in a way that really works), light mystery, and interesting use of multiple cultures' folklores, this book proves that not only is its original a good story, but it can handle a serious expansion of its world as well.