Kate Milford’s latest novel, “The Thief Knot,” is part of the Greenhouse Glass series. The books are set in the fictional city of Nagspeake, a word she created from two places she vacationed: the Nags Head and the Chesapeake Bay. (Houghton Mifflin Harcourt, Daniela Volpi Photography)

“As a kid, I was a lot like Marzana,” said Kate Milford of the main character in her new novel “The Thief Knot.” “I wanted to solve a mystery so badly. My friend and I used to ride our bikes through the neighborhood, looking for anything suspicious.”

At the beginning of the book, Marzana is sure that nothing interesting ever happens in her part of a fictional city called Nagspeake. But soon she’s embroiled in a strange mystery — one involving a kidnapped girl, a dubious substitute teacher, sneaky knots and secret tunnels.

Secret waterfront world

In real life, Milford never stumbled on such a mystery — so she decided to create her own. Early efforts starred her pet rabbits as detectives.

Then as an adult, about 13 years ago, Milford began a project for fun. She started making up the history and geography of a city she named Nagspeake. At her husband’s suggestion, she created a website so others could enjoy her pretend city.

Nagspeake was inspired by the waterfront places she’d known all her life. Milford set the city on a bay, similar to the Chesapeake Bay. She drew from her childhood in Riva, Maryland, close to Annapolis, and from visits with her grandmother, who lived on the Chesapeake, near Sandy Point.

The name of the city is a combination of “Chesapeake” and “Nags Head,” where Milford often vacationed in North Carolina with her parents and three younger siblings.

“I loved creating this world,” she told KidsPost by phone. “I had so many memories to draw from: walking on the beach as a kid, taking out the little boat.”

Milford soon began writing books set in Nagspeake. In these tales, nothing is as it may seem. There are hidden identities, concealed rooms, smugglers and a ghost. “The Thief Knot” follows two earlier novels in Milford’s middle-grade series: “Greenglass House” and “Ghosts of Greenglass House.” She has written other novels for kids ages 12 to 14.

Each book can be read and enjoyed on its own. “The Thief Knot” tangles its young detective, Marzana, in a suspenseful adventure.

Milford had always been drawn to creating imaginative worlds, she said. As a kid, she liked to erect elaborate forts for her collection of tiny animal figures.

She can now see this passion in her two young children. At their home in Brooklyn, New York, during the coronavirus outbreak, “we’re making a lot of forts,” she said with a laugh.

A quiet type of kid leader

“The Thief Knot” is full of puzzles and surprising twists. It’s much like Milford’s favorite childhood book, “The Westing Game,” by Ellen Raskin, and the stories her dad told on long family car trips.

Besides a love of mysteries, Marzana and Milford share another trait. They often feel anxious in social situations, Milford said.

To find the kidnapped girl, Marzana must deal with her anxiety. She needs to put together and lead a team of kids with skills very different from her own, including a magician, a code expert and a ghost.

The usual leader in a kid’s book is extroverted and confident, a take-charge type. Quiet Marzana likes to plan and strategize.

“I wanted to make space for a different type of leader,” said Milford. “And to show why her crew might want to follow her.”

Follow her and help to unknot some of Nagspeake’s complex secrets.