Tess Gerritsen doesn’t see her latest novel as a risk. Instead, she’s just following her muse.

Through the years, the Camden mystery author has developed a cast of memorable characters, including Boston police Detective Jane Rizzoli and medical examiner Maura Isles.

Rizzoli doesn’t even appear in Gerritsen’s new Ballantine novel “The Bone Garden,” while Isles makes what could charitably be called a cameo appearance.

Gerritsen knew she was taking a chance by jettisoning such familiar characters.

“Absolutely [I was concerned],” she said in a phone interview from home, in a break from her national book tour. “I had a lot of readers who said, ‘We just want to read Jane Rizzoli books.'”

But Gerritsen, a history buff, writes about what she finds fascinating.

“I can only write about subjects I’m interested in,” she said. “It has more to do with a sense of curiosity. And when you research all this stuff, you want to put it in a book.”

More specifically, “The Bone Garden” is set in the 1830s, the dawn of microbial theory in medicine.

“I wanted to find out what made Oliver Wendell Holmes advise doctors to wash their hands [between patients],” she explained. “I wanted to explore what kind of man he was and what he might have seen to give him this revelation.”

What drew Gerritsen to this dark time in medical history?

“I think it was the horror of the era that caught my attention,” she said. “I found out what it was like being a medical student back then, and how uneducated a lot of them ended up being. There were diploma mills that were turning out butchers. The poor had to dig up cadavers to dissect if they couldn’t afford to buy them. You were better off not going to a doctor, because they killed more than they cured.”

“The Bone Garden” begins with Julia Hamill, a recently divorced teacher, digging up some very old bones from the area where she hoped to plant a garden on her suburban Boston property.

Next Gerritsen takes readers back to the 1830s, where they meet Rose Connolly, a recent immigrant seamstress from Ireland in her late teens, and Norris Marshall, the talented but penniless medical student with whom she falls in love. Their tentative romance is set against the backdrop of a Boston under siege from the West End Reaper, a serial killer. Holmes is one of Marshall’s rich classmates at Boston Medical College. (While Holmes was the only real-life character in the book, Gerritsen drew some character names from lists of people who had been executed in Massachusetts during that time.)

The novels cuts back and forth from the present, as Julia attempts to uncover whose bones she found, to the past, where Rose and Norris, witnesses to the Reaper’s atrocities, fight to stay alive.

Gerritsen has been pleased with the responses she has been getting from her readers for “The Bone Garden,” which was No. 10 last week on the Publishers Weekly best-seller hardcover fiction list.

“I’m getting some of the best fan letters I’ve ever had,” she said. “People are reluctant to pick up a historical novel, but they end up surprisingly happy.”

Many of her fans will be glad to know that Gerritsen is working on another Rizzoli-Isles novel, due out at this time next year.

What’s the biggest lesson that Gerritsen learned while researching “The Bone Garden”?

“Medicine’s always changing,” she said. “The things we’re doing today will be a shock in 100 years. Science never stands still.”

Tess Gerritsen will be doing a reading and book signing at 7 p.m. Tuesday, Oct. 9, at Borders in Bangor (990-3300). For information on the author, visit www.tessgerritsen.com.