Hallie Ephron's mysteries are so inventive and seductive it seems she's tapped into some formula whereby bestselling books effortlessly appear as if by magic.

Consider her latest, titled "There Was an Old Woman" (William Morrow). Evie Ferrante, a young museum curator taking care of her terminally ill mother's home, is drawn to help an elderly neighbor named Mina Yetner - who either suffers from increasing dementia or is the victim of sinister manipulations that seem to have increasing implications for an entire distressed neighborhood.

It's a terrific and instantly engaging plot - but along with the tightening garrote of tension are wonderfully developed characters who deal with poignant problems including alcoholism, fraying sibling and maternal relationships, care for the aging, and the tantalizing hope that, despite evidence and Thomas Wolfe to the contrary, you CAN go home again.

Ephron is the guest of honor at a luncheon/signing Tuesday in Mystic's Bank Square Book Store, and she'll discuss "There Was an Old Woman" along with the writer's life and her path to success.

Her seemingly alchemical formula, though, is actually the result of hands-on experience, a steady work ethic, and the ability to learn from her own mistakes. By now, Ephron has authored three novels, cowritten the five-book Dr. Peter Zak mystery series with Donald Davidoff, and penned three nonfiction books including the still-popular how-to guide called "Writing and Selling Your Mystery Novel."

"The first novels were a series with a partner, and they were very traditionally structured," says Ephron by phone from her home near Boston. "You throw the reader off his or her equilibrium with the first chapter, toss some clues out and develop the characters and plot as you go along, and gradually reveal secrets."

Those books, and several years spent as a high school teacher, gave Ephron sufficient confidence that, when approached by a publisher to write a how-to book about mystery novels, she accepted.

"As a teacher," she says, "it's a pleasure to show students how to break things down. You never really know how to do something or how something works until you break it down. And that ability to analyze was a big help with 'Writing and Selling Your Mystery Novel.'"

That book is still in print and very successful, Ephron says, and the source material is a constant reminder to herself now that she's embarked on a post-collaborative career that includes, along with "There Was an Old Woman," the mysteries "Never Tell a Lie" and "Come and Find Me."

But it doesn't mean it isn't hard work- both from a physical and mental perspective. One of the main inspirations for "There Was an Old Woman" was the fact that Ephron, now 65, was going through anxiety about aging and death. Among other things, she lost her sister, the screenwriter/journalist/director/playwright Nora Ephron, in 2012. (Her two surviving sisters, Delia and Amy, are also screenwriters.)

"I do think that to be a good writer you have to address your biggest fears," Ephron says. "It doesn't mean you write it exactly the way you're going through it, but you channel the fears into a fictional plot. I do read the obituaries every day, and I do take note of who was older or younger than me.

"I think about dementia, too, and it occurred to me how easy it would be to make someone believe they were losing their mind. That's a plot device, but the underlying fear is maybe that you're going to be a burden to other people - your family or children - because you're unable to care for yourself mentally or physically."

In the novel, Evie's alcoholic mother enters the end-stage of the disease and is placed in a hospital intensive care unit. Meanwhile, Evie is as astounded by the disarray of her childhood home as she is by her own guilt; preoccupied with her job, she's ignored her mother and left a lot of care and responsibilities to her sister.

Trying to take up the slack, Evie sets out to clean house and discovers unexplained piles of money and signs of wealth well beyond her mother's means. As she increasingly turns to Mina and an old childhood acquaintance, Finn, for help, Evie becomes ensnared in puzzling scenarios wherein various relatives and neighbors might not be as they appear. If there's a dark purpose behind increasingly violent events, Evie's got to figure it out even as Mina's own mental uncertainty reaches a crucial point. At the same time, Evie must reconcile with her sister to help make their mother's decline and passing as comfortable as possible.

The book works on many levels and is at once tense, creepy, bittersweet and real. It takes a lot to juggle those elements and, for all her knowledge of craft, Ephron cheerfully admits there's an ongoing element of discovery when she works.

"A lot of what happens evolves over the course of writing the book," Ephron says. "I might discover a familial connection that wasn't there." She laughs. "I don't know how it happens. Or I'll get to a point where I'm stuck. I know what's going to happen in 30 pages but not how to get there. It's like untying a knot, and I'll back up and try increasingly ridiculous 'what if?' scenarios. It's an ugly process."

Obviously, as per the success of her books, Ephron always figures it out, which she describes as "really exciting," if typically frustrating.

"Usually, I'll 'get it' at the point when I'm buying chicken or taking a bath and don't have a pen or anything to write on," she says.

And if the process was unnerving at first, Ephron - teacher and veteran writer - no longer panics.

"I've been in that position so many times. I know, now: you keep beating your head against the wall and you know it will eventually happen."

Hollywood ending

In 2011, Ephron's novel "Never Tell a Lie" was made into a Lifetime television movie entitled "And Baby Will Fall." Since all three of her sisters have had careers in the film business, Ephron was delighted on more than one level.

At the same time, she had no expectations that the film would be faithful to the novel.

"I grew up in Hollywood and had no illusions that anyone would do anything other than pay me," she laughs. "The movie ended up nothing like the book, and the ending is completely different. But I was just very excited to have it adapted and was thrilled they wanted to do it."

IF YOU GO

Who: Writer Hallie Ephron

What: Discusses and signs copies of her new mystery, "There Was an Old Woman," at a luncheon in her honor

When: Noon, Tuesday

Where: Bank Square Books, 53 West Main St., Mystic

How much: $26 includes lunch, choice of wine or sparkling water, a signed first-edition of the book, and a Q&A with the author.

For reservations or information: (860) 536-3795, banksquarebooks.com