Erin Hart's new mystery is about shape-shifting and duality, lies and truths, seals that turn into maidens, people who might not be what they seem. Nearly every character has a secret, which makes "False Mermaid" a wonderful page-turner filled with red herrings and head scratches.

Hart, a former nonfiction writer who now writes novels, is a bit of a shape-shifter herself. With a first name of Erin, a County Offaly husband named Paddy O'Brien, and three mysteries steeped in rainy bogs and fiddle music, you might think she's as Irish as a shamrock. But, like most of us, she's more of a mutt.

"I'm German and Belgian and English and Irish, and my German grandma always thought I was way too Irish," she said, pouring a cup of tea and buttering a scone. She was sitting at a sunny table in her St. Paul living room, a cheerful space with yellow walls and a basket of peat and a fiddle resting on the fireplace mantel. "Nobody else in my family is as Irish as I am, you know?"

Ireland grabbed her heart from the time she was a child -- who can explain it? -- and she's been going there for 30 years, traveling to music festivals, studying the language, doing a bit of singing and a lot of dancing.

In recent years, though, since she started writing novels, her Irish travels have also included visits to archaeological sites and museums as she researches archaeology, geology and mythology. Her books are grounded in the country's land, history and music, and Hart, 51, is dedicated to the details, down to the way dirt flies during a peat storm, or the way bog water changes the color of human skin.

(Right now she is researching the ingredients of ninth-century ink. You just never know where a story is going to take you.)

Scary St. Paul neighborhoods

It was the bogs that grabbed her, handing her a story on a platter, she says, when she heard about a turf-cutter in Sligo who discovered the 350-year-old head of a woman. "My immediate first thought was, 'That's a mystery!'" she said, and indeed it was, becoming the heart of her first book, "Haunted Ground."

At the time, Hart was a theater critic who had published just one piece of short fiction. The thought of writing a novel terrified her -- and then she decided that what scared her should be attempted. The book ended up selling in 12 countries. ("I'm big in France, apparently," she says.)

"False Mermaid," which publishes this week, is set both in Donegal and in St. Paul, as her protagonist, Nora Gavin, returns home from Ireland to solve her sister's murder.

"That murder was part of the back story of the first two novels," Hart said. "And I knew that eventually I'd have to tell this story -- which meant I'd have to figure out for myself what happened, because I didn't really know.

"It was a real challenge. It took me a long time to write it, (a) not making it a soap opera Lifetime channel movie, and (b) writing about that which is exotic in St. Paul, Minnesota, making it as exotic as Ireland."

In Hart's hands, lovely Hidden Falls Park along the Mississippi River becomes a dark place, with human skulls and secrets buried in the mud (and will anyone ever go there alone again after reading this book?). Frogtown is dangerous, the Central Library feels creepy, and even an ordinary parking garage near Mears Park takes on an ominous isolation.

"All of those places are sort of otherworldly," she said. "So I brought in all these bits of mythology that had to do with the other world, as well -- the story of Persephone, and the selkie myths."

The myth of the selkie -- a seal that takes on the form of a human woman -- winds through the novel and is at the heart of an old tune that helped inspire the plot. "An Mhaighdean Mhara" ("The Mermaid") is about a selkie who marries a man -- and then vanishes. The song claims she returned to the sea, but some believe her husband killed her.

At her book party on Friday, Hart will sing the song.

"In Irish," she said, and shivered.

What if?

Hart's writing method is to "sit down and stare into space, usually for three or four hours, until Paddy gets up and starts watching CNN and I have to go do something else."

During those hours of staring, she goes over "what-ifs" in her head, spinning out scenarios, jotting character sketches, worrying over place names, listening again and again to the song that infuses the book, to let it "kind of worm its way through my brain, and inspire."

Her next book, which she has just begun, will be set once again in Ireland, back in the bogs. "And the little piece of true history it's based on is that a book turned up in a bog in Tipperary, a book of psalms from the ninth century," she said. "And it got me thinking.

"What if it's not only the book that's in the bog, but maybe the guy who made the book? What if it's not a book of psalms, but there's another book they didn't find?"

She perched her chin on her hand and stared off into space, mock-musingly. "Hmmmm. What if?"

Laurie Hertzel is the Star Tribune books editor. She is at 612-673-7302.