New York Times bestselling thriller novelist Ace Atkins is a shape shifter. There's really no other way it could work.

Half the year, as the estate-chosen author to continue writing the late Robert B. Parker's Spenser series, Atkins is all Boston-centric — marinating physically and/or mentally in the Red Sox, Hah-vad and Southie, Harpoon and Sam Adams, and adjectives like "wicked."

The other six months, working on his own (very different) Quinn Colson novels, Atkins is back home in Oxford, Mississippi, with his wife Angela and sons Jeff and Billy and a coterie of rescue dogs — enjoying fried catfish and pork ribs, Bill Faulkner and Jim Burke, and an iPod full of Elvis and the Drive-By Truckers.

The stylistic differences between Colson and Spenser are just as dramatically different. The books about the former, an ex-Army Ranger now trying to keep order in a rural, crime-infested Mississippi county, are stunning exercises in darkly literate noir, punctuated with bottle rockets of humor. With Spenser, Atkins must not only replicate Parker's spare and perfect rhythms, droll punch lines and the intertwining narratives of much-loved characters, he's got to e'er evolve the series into the future.

For the next two weeks, Atkins — who, it must be said, is a friend of mine — will be in full Parker mode as he heads out in support of his fifth Spenser novel, "Robert B. Parker's Slow Burn," wherein three Boston firemen are killed fighting a blaze in an old church. There's no definite cause but, as more and more buildings burn, it becomes obvious someone is setting fires with a frequency that suggests escalating psychosis. Spenser is brought in by a surviving firefighter who believes not enough is being done to stop the blazes or figure out why they're happening.

The third stop on Atkins' "Slow Burn" tour takes place Wednesday with the latest "Beer & Books" event at the Cottrell Brewery in Pawcatuck.

"It's very, very different writing the two styles," Atkins says from Oxford last week. "After balancing Spenser and Quinn for a few years now, it actually has gotten a bit easier. It's like a musician who plays classic jazz one night and the next he's working with an outlaw country band. And there are players who do that. You get better at it as you do it."

Atkins says that, by this point, he can settle into the flow and characters pretty quickly once the stories get rolling.

"The transition is the toughest part," he says. "I'm doing two books a year — one for each series — and to put the brakes on Spenser and suddenly have to go right to Quinn can be abrupt. But I've got a process. For Spenser, I'll go to Boston and get a sense of place and just listen to folks talk. I go to Fenway and eat hotdogs and drink some cold beer and then re-read some of Bob's books and all of a sudden I'm good to go."

As for segueing back into Colson Mode, Atkins laughs. "Well, in Mississippi, I just wander around the hunting department at Wal-Mart and I'm instantly back."

Another significant difference faced by Atkins is pretty obvious. Colson is his own creation. And despite very popular series with Sunny Randall and Jesse Stone, Parker's Spenser is arguably the most indelible private eye in the annals of detective fiction. Carrying that tradition forward can involve delicate machinations.

"When I wrote my first two Spenser books ("Lullaby" and "Wonderland"), the whole goal was to keep Parker's work going and keep the character alive in fans' minds," Atkins says. "It was the same Spenser. But, six years after Bob's death, it's kind of a strange transition period. I have to keep the series current. The Parker family gave me free creative rein, but they did ask that I keep the character contemporary.

"Moving him into the 21st century is tough. I have to take liberties. Spenser has to move forward, but maybe not in real time. He and Susan and Hawk are aging, but they're not 85. For me as a writer, I have to believe in this stuff. Spenser's probably in his mid-50s and that enables me to show a progression and the passing of time. I want readers to see things change, that I'm not just going to tell the same story over and over again. "

Atkins also loves to throw bits of minutiae into the new novels, too. In "Slow Burn," for example, Spenser takes Susan for a brief getaway to Cape Cod and the hotel where they first fell in love during the fourth Spenser adventure, "Promised Land." It's where Hawk first appears, too — as a foe — and there were hints that maybe Parker suspected the long-term relationships of these three characters.

"It's fun for me to do these little touchstones to earlier works," says Atkins, who in college collected Parker first editions the way frat boys play beer pong. "Maybe one in a thousand readers might recognize the old references, but it's my way to maybe connect the past and the future."

For "Slow Burn," it occurred to Atkins that, for all the Spenser cases Parker wrote about the nuances of Boston, he never wrote a book about the city's fire department.

"It's a huge part of the culture and it's a big family and very much a big part of the city," Atkins says.

He came up with a basic story about an arson fraud case and went to work. After getting fairly far along in the writing, Atkins went to Boston and met with several officials and firemen in the Boston Fire Department.

"They couldn't have been nicer to me," Atkins says. "I met with the commissioner and the arson investigation unit and was feeling pretty good about things — until I described my plot." He laughs. "They said, 'Oh, that would never happen that way,' and I left Boston with basically no story."

Undaunted, Atkins started researching the Boston Fire Department and discovered an outbreak of arson in the early '80s. Hundreds of fires were started and, ultimately, investigators caught an entire ring of arsonists.

"The thing was, they thought they were actually supporting the fire department because their budget had been slashed and they were using outdated equipment," Atkins says. "The arsonists just wanted to create a situation where the department would get more funding. They were, like, fire department groupies — but the knuckleheads just spiraled out of control."

During downtime on the "Slow Burn" tour, Atkins will look over the proofs of "The Innocents," the new Colson book, which is due out in July. He's also just finished co-writing a screenplay based on the inaugural Colson story, "The Ranger."

"I'm really excited about 'The Innocents' and the possibility that the screenplay will get made into a film," he says. "I think it's time the series moves to the next level. I'm keeping my fingers crossed."

He also says he's heard very good things about the Cottrell Brewery, and that the "Books & Beers" series is a truly great idea.

"Beer is always good," he says, "and really good beer is better. I'm looking forward to being at the brewery because writing the Spenser books and getting to talk to fans about Bob Parker is always fun. I've always idolized him, and I basically see myself not so much as a writer in this case but more like an ambassador."

IF YOU GO

Who: New York Times bestselling novelist Ace Atkins

What: Signs copies of his latest Spenser novel, “Robert B. Parker’s Slow Burn,” at the latest “Beer & Books” event co-sponsored by Bank Square Books and the Cottrell Brewery

When: 6:30 p.m. Wednesday

Where: Cottrell Brewery, 100 Mechanic St., Pawcatuck

How much: Free and includes beer tastings; copies of the book available for purchase

Info: (860) 536-3795