MIKE COOLEY has the voice of authority. While Patterson Hood is presumably the mouthpiece of the Drive-By Truckers—the genre-bending Southern rock band the two guitarists founded in 1996—Cooley is the heart of the band, his cut-to-the-quick songwriting responsible for grounding the sometimes lofty concepts of his bandmate. While Hood arcs a throughline of a murdered small-town preacher's wife through several songs on the band's latest collection, Go-Go Boots, Cooley's contributions are unpretentious, stand-alone country songs sung in a deep, matter-of-fact baritone. "Everything's in there," he says of his songs' comparative simplicity. "Or I hope it is. If it's not, I obviously didn't want to say it."

Despite having played together off and on since 1985, when they formed Adam's House Cat, Cooley and Hood have never sat down to write a song together. "It's gotten less volatile for sure," Cooley says of their sometimes-contentious relationship. "As far as the writing thing goes, we have probably planted ideas in each other's heads but we've never actually thought about the writing of a song together. For me, it's something I prefer to do by myself." He adds, "It's one of the only things I still get to do by myself."

Nine albums in, Go-Go Boots is among the Drive-By Truckers' mellowest recordings. Following the massive power-chord anthems that populated 2001's breakthrough Southern Rock Opera, Drive-By Truckers have continually expanded and refined their sound, often by incorporating the songs of other members in the band. The slower ballads of guitarist Jason Isbell opened a fresh door during his run with the band in the mid-'00s, and bassist Shonna Tucker (Isbell's ex-wife) has been contributing her own material since 2008's Brighter Than Creation's Dark. Indeed, Tucker's move to the frontline is the most noticeable evidence of the shift in the band's approach, her femininity serving as a counterpart to the overdriven riffs that at one point seemed to be the hallmark of DBT's style. It could be said that the band is approaching their Rumours era; not unlike Fleetwood Mac, the burly, bloozy, guitar-driven work that earned Drive-By Truckers early notice has given way to a softer sound. Now the girl in the band is singing, and the band is more popular than it's ever been.

Go-Go Boots displays a nimbler, more dynamic sound that's equal parts country and soul. It's the fruition of a lifetime of being deeply influenced by the musical history of Muscle Shoals, Alabama, the area where Cooley, Hood, and Tucker grew up. Hood's father, David Hood, was a session bassist who played on many of the seminal soul recordings to come from the FAME and Muscle Shoals Sound Studios in the '60s and '70s. That influence has always been deeply embedded in the Truckers' DNA, but it wasn't until now that the band explicitly explored it on record.

"We've always liked that stuff, but we just kind of naturally gravitated to our own take on it," Cooley explains. "It had to be natural. And the key to being able to play that kind of stuff is to not force it. It's got to feel real. And I think we finally got to a point where we could do that." Recent time serving as backing band for both Bettye LaVette and Booker T. Jones helped. "That made us a lot better in every way, particularly when it came to approaching some of these kind of sounds and even writing some of those kind of songs. It takes a little more finesse and patience to do that kind of stuff than to just rock out."

Go-Go Boots reveals that Drive-By Truckers have reached a stable point in their evolution; in fact, for the first time since 2004's The Dirty South, the band sounds fully gelled. While Go-Go Boots isn't as good as 2003's impeccable Decoration Day—indeed, one of the best records of the past decade—its highlights are unexpected peaks in the Truckers' catalog. The band's confidence and flexibility can best be exemplified by the album's inclusion of two covers of Muscle Shoals musician Eddie Hinton, whose songs the Truckers play over the PA before their live shows. One of them, "Everybody Needs Love," is an unironic, uplifting anthem that's the happiest thing Drive-By Truckers have ever recorded, a gospel-pop song that's surprisingly light on its feet.

Despite the changes in their sound, the Drive-By Truckers seem to wallow effortlessly in integrity, and even sometimes seem to shoulder the responsibility of upholding a certain musical history—not just of Hinton, but of Muscle Shoals, perhaps even the entire South. But Cooley says, "I don't know if it's a responsibility. If it happens, it happens. But that's not part of the mission statement."