It is Hagen's first book, and she'll sign copies at 7 p.m. Friday at the Doylestown Bookshop on Main Street.
The case invoked fear in the hearts of city officials here, not to mention parents everywhere, because Philadelphia was primping to host the Centennial Exposition of 1876.
And it prompted a nationwide manhunt.
With creep-inducing clarity, Hagen describes a criminals' lair in New York's notorious Five Points and a suspect with an injured finger, the end of which was withered to a bony point.
Inquirer reviewer Bill Kent praised Hagen's book as "an elegantly told, superbly accomplished history of good and bad intentions gone awry," and suggested it may inspire "new walking tours of Germantown and South Philadelphia."
We've cornered Hagen, 34, at Mugshots coffee house on Fairmount Avenue, where she writes when the nearby condo she shares with her husband, Jeff, seems suddenly small. If half the writers tapping on keyboards in this place were to publish books, Borders could come back from bankruptcy.
Hagen smiles easily and often, cushioning the impact of anticipated criticism with self-deprecating humor:
"My undergraduate degree was in education," she says. ". . . and as soon as I started teaching, I realized I didn't know anything."
Teaching English lit at Council Rock High School North by day, Hagen stayed one step ahead by night, earning a master's degree in that subject at the College of New Jersey. Later, in 2009, she graduated from Goucher College in Maryland with a degree in creative nonfiction and a first draft of this book.
Hagen had decided early on in her six-year process to write about the Germantown neighborhood where her father was raised. On one of many research missions, she stumbled upon news reports of the Ross kidnapping and couldn't shake the eerie tale.
Not to spoil the story, but Hagen calls the case "one of history's mysteries."
One account of the Ross case suggests that every parent's admonishment, "Don't take candy from a stranger," grew out of this kidnapping because Charley Ross was indeed lured away with the promise of candy.
Hagen says she heard the same warning as a happy-go-lucky child in Florida, Indiana, and, finally, Levittown.
She also recalls her mother's account of a 1983 incident:
"I would have been in first grade," Hagen says. "I was standing at the bus stop, about 20 feet from my front door where my mom was standing.
"As my mom tells it, she saw a man get out of a dark car and walk toward me so she screamed at the top of her lungs: Get away from my child!"
"I remember none of that," Hagen says. "But you're kind of blinded when you're growing up."
Hagen is on leave now from her teaching post, living, she says, "the life of a starving artist."
"The first day I was off the payroll, my dryer and my refrigerator broke and I totaled my car."
Still, she's at work on her next book - another nonfiction narrative, set somewhere in this country's past.
"The stories that interest me most have been lost to history," she says, "overshadowed by more famous events."
Contact Inquirer staff writer Dianna Marder at 215-854-4211, firstname.lastname@example.org, or @marderd on Twitter. Read her recent work at http://go.philly.com/diannamarder.