NEW ORLEANS - Say a guy tells you about a guitar player you've never heard of. The guy says, "He's probably the most underestimated musician on the planet and also probably one of the most advanced."

Your first reaction might be, "Who does that guy think he is, Eric Clapton?"

In this case, the answer is yes: The guy telling the story is Eric Clapton, speaking about slide guitarist Sonny Landreth, who comes to Rhode Island Sunday to play the Rhythm and Roots Festival in Charlestown. Landreth, a 52-year-old Louisianan, is one those musicians who has been working steadily for decades, earning a devoted following among his peers, if not the public.

Clapton, who relayed his comments through his assistant, Cecil Offley, isn't alone. Ask Warren Haynes, guitarist for the Allman Brothers and leader of Gov't Mule. To him, Landreth is nothing less than "truly great. ... He has a unique slant on slide-guitar playing, in that he has come up with some different tunings and ways of playing them that give him his own voice."

Or take Quint Davis, who for decades has been booking hundreds of acts annually as chief of the New Orleans Jazz and Heritage Festival. "I was told by another guitar player one night during jazz fest [in May] that ... other guitar players can't really take away anything from what [Landreth] does," Davis says. "It's so different and so unique that it almost doesn't translate."

Apart from when Landreth purses his lips or smiles a little, his face is quiet when he plays, but his hands are never so. He picks. He strokes. Sometimes his right pinkie is a mallet, striking the cords for a percussive effect. Other times, it's a bow, sliding across them. He doesn't play only the strings, either: He'll tap or even lightly punch the body, or get the sound he wants by nudging the back of the neck up high while his slide hand is working down low.

That slide hand is what makes Landreth special. All slide players get their sound by striking the strings with their right hand while moving a slide - sometimes just a clear plastic cylinder placed over the pinkie - along the guitar neck. Unlike other guitar styles, in which left-hand and finger positions produce chords, the strings never touch the frets, and tones are governed chiefly by slide location. Landreth, however, does both: While the pinkie slides, the other three fingers are playing chords.

Besides technique, what informs Landreth's playing and songwriting is geography, or, more specifically, the rich Cajun atmosphere of southwest Louisiana. He's not a Cajun - he was born in Canton, Miss., in the hospital said to have sent slide legend Elmore James into the world - but his dad moved the family to Lafayette, La., when Landreth was 7, and he is accepted as a local. Cajun mainstay D. L. Menard told the New Orleans Times-Picayune last year, "He's just about the best we've got around here. It's not zydeco and it's not Cajun, but it fits right in."

If you had to name it, you'd say that Landreth plays the blues, with strong zydeco colorings he inherited from Clifton Chenier, zydeco's undisputed king. Landreth met him when he was 16 or 17, and he was asked to join Chenier's Red Hot Louisiana Band band, becoming its first white player, a bit more than a decade later. Landreth still thinks of Chenier as his greatest musical influence, even as he's gone on to record with bandleaders from John Mayall to Jimmy Buffett, who thinks Landreth's playing is "as good as it gets." Today, besides touring with his own three-piece band, Landreth most often plays with the Goners behind his good friend, John Hiatt.

Landreth's solo work has a checkered past. In the early '70s, he hooked up with the notorious Huey Meaux, between the accomplished record producer's trips to prison. For reasons Landreth never understood, Meaux held onto what would have been Landreth's first album for years. It was finally released as "Prodigal Son," with other music, during Meaux's second incarceration.

Until the past couple of years, there was a long stretch when Landreth would appear in New Orleans only once all year, during the Jazz Festival, prompting Davis to regard him as a bit of a recluse. His recent resurgence there - he appeared more than a dozen times during the two weeks of the fest this year - has impressed Davis, who exercised "executive privilege" at this year's fest to personally introduce Landreth's gig. He proclaimed him "Louisiana's new guitar legend."

How can a 52-year-old be new?

"He's not new, of course, but I think his legendariness on a much broader scale is new," Davis says. "He played as many gigs around town during Jazz Fest this year as he's played combined in the last 10 years. And so that's an emergence. Of course, all these overnight successes have been at it for 20 years."

Even if his latest album, "The Road We're On," might represent the best guitar work of the year, Landreth professes no expectation of high-profile success, overnight or otherwise. "Let's face it," he says, "I'm 52. I'm not in the demographic for mainstream radio, and we definitely fly below the radar anyway."

Of course, that depends on who's sitting at the screen: "People who recognize slide guitar place Sonny in the top few in the world," Haynes says. "That doesn't make him a household name, but a lot of true musicians never get that household status."

Steve Morse of the Globe staff contributed to this story.