“I’m in show business,” Costello says. “If I want to wear big glasses and a hat, I will.” Photograph by Jeff Riedel

In some respects, Elvis Costello is the sum of his songs—his “oeuvre,” as he once mocked a BBC interviewer for saying—but he is also a compilation of encounters with other musicians. Athletes chase wins, and bankers thrive on deals; Costello hungers after collaborations, which can then be processed into their by-products: recordings, friendships, gate receipts, ideas, and anecdotes.

Some people have him frozen in Lucite as the skinny, sneering, knock-kneed rocker with the Buddy Holly glasses and the New Wave suits who, in punk’s wake, in the late seventies, unleashed a series of furious, lyrically tricky but not uncatchy albums and singles that still pop up on classic-rock radio or transgenerational playlists. Or else they are dimly aware of a restless and protean figure who amid the ripening of a career sampled, and often mastered, other genres and styles—a man of many talents and a few excesses. They may think he was authentic once and pretentious later. Among those who have paid attention the whole way, he may be best and most properly known for his stamina as a performer and his enduring prolificacy as a songwriter. “He can toss off an album in an afternoon,” the producer T Bone Burnett, with whom Costello has made a few, says.

Yet his peers, if there can be said to be any, may consider him the industry’s highest-ranking music nut; he’s as much a fan as he is a participant, and his participation is relentless. He has evolved into one of the most spirited accomplices in tribute gigs, variety evenings, and extracurricular combinations. His enthusiasm for the work of others is now so deep and wide that his calendar strains with far-flung one-offs and barely compensated commitments. A jolly “Penny Lane” for Barack Obama and Paul McCartney at the White House, last June; a sly turn, last month, as master of ceremonies, and a warm performance of “Brilliant Mistake,” in a limited-run T Bone Burnett revue featuring, among others, John Mellencamp, Ralph Stanley, Leon Russell, and Elton John; perhaps a pensive rearrangement of “(What’s So Funny ’Bout) Peace, Love and Understanding” when he performs in December at the Oslo Spektrum, in honor of the Nobel Peace Prize. Costello has worked himself not just into the company of presidents and knights but also into the network of musicians’ musicians and songwriters’ songwriters, the confederation of lesser-known liner-note hot shots who are rarely separated by any more than two or three degrees.

He is too guileless a buff to be a snob, but he is discerning. In his younger days, he said nasty things about other acts, and still scoffs at a few that some hold dear. “I am really grossly offended by Led Zeppelin,” he said in 1986. “Not only because they’re total charlatans and thieves, but because it actually embarrasses me.” He is unkind to the eighties. Once an avid creator of mix tapes and now a mischievous e-mailer of obscure MP3s, he’s as canny as anyone at choosing songs to cover; he has a record geek’s taste for the B side as well as a curator’s love for the big ones. He doesn’t shy away from “My Funny Valentine.” Each episode of “Spectacle,” his talk-and-jam cable-television program, demonstrates his pluck at taking on every section of the songbook, and in explaining why.

Conversation with Costello can veer in many directions. He knows a lot about a lot. Still, it inevitably fixes on his musical influences and alliances, and the circumstances leading up to them, and the opportunities and insights arising out of them—the recollection of each one opening, like a trapdoor, into a vast underground of other projects and encounters and innumerable chambers of musicological erudition. He’ll talk your ear off. He’ll drop some names. He’ll say, “And Schubert, I mean, really. How great is Schubert? I mean, come on! Five hundred fucking songs!” It can sometimes seem that musicians live in a parallel universe populated only by other musicians and maybe a few alien non-musician life forms who, by virtue of their musical knowledge and interest, can breathe the air. There is no geography in this universe. There are no homes. There are just songs. You live in your own and visit others’.

One evening last spring, Costello, who is fifty-six, found himself in the wine country of Northern California, as the closing act of the Sonoma Jazz Festival, which was more or less jazzless; he was preceded by Crosby, Stills & Nash, Earth, Wind & Fire, and the Neville Brothers. He was the only one with any new material to speak of. He had flown in a group that has come to be called the Sugarcanes—six of Nashville’s finest acoustic players, with whom he’d released one album, in 2009, and just recorded another, out this month. He has been touring with them, off and on, for more than a year. For the Nashville guys, it was good work: they got to back an ace and play fresh, tasty material, for rock-and-roll pay. For Costello, it was a chance to perform his old songs with a crack ensemble in a traditional country-bluegrass context that had sometimes been the one in which they’d been conceived, if not originally executed. He also got to write new songs that suited the group well.

In a field backstage, there was a long, narrow tent divided into compartments, each of which, by way of a folding table, a clothing rack, a name on a piece of paper, and a plate of fruit and cheese, had become a dressing room. Costello got one, and the band got another. He had come up from San Francisco separately, in a chauffeured S.U.V. (“I am not going to talk all the way through, because of the voice, you know,” he’d said, on Lombard Street, and then he’d talked all the way through.) He wandered in and out of the band’s stall. Conversation bounced from bluegrassy personage to bluegrassy personage, as though from verse to verse, the unsung chorus being something along the lines of “The world has forgotten more than you will ever know.” They segued from the New Lost City Ramblers (“They had sleeve garters!”), Roland and Clarence White (“Roland was there when Clarence got killed”), Spade Cooley (“Was that before or after he killed his wife?”), Richard Bennett (“He wrote ‘Forever in Blue Jeans’ with Neil Diamond”), and John Hughey’s double-necked pedal steel guitar (“He called the country neck the ‘Kroger neck,’ ’cause that’s what bought the groceries”). Costello soaked it all up, chiming in when he knew his business.

Robbie McLeod, his longtime tour manager, showed up and handed out cash for expenses—forty dollars a day for each musician. “Living the dream,” Jeff Taylor, the accordionist, said. In the background, you could hear that the Neville Brothers had taken the stage. Costello—dressed in black jeans, a black hooded sweater, a coral-colored cashmere scarf, and what you might call a duck-hunting cap—pointed over his shoulders with his thumbs, and he and some of the guys hustled off to watch the Nevilles work their way through “Fever” and “Hey Pocky Way.”

As the Nevilles’ set went on, the Sugarcanes drifted back to the tent and changed into their party shirts. Mike Compton, the mandolin player, put a jacket on over his overalls. Jim Lauderdale, who plays guitar and sings harmonies, stood out in the field, in a cowboy shirt, doing Tai Chi. Pete Thomas, the drummer in Costello’s foundational but defunct band, the Attractions, as well as its successor, the Imposters, and now performing with the Sugarcanes for the first time, tapped on a table with his drumsticks. After a while, Costello, too, disappeared into his stall to change. He emerged in a shiny silk midnight-blue suit, black patent-leather Chelsea boots, a blue shirt with white pin dots, a black-and-blue polka-dot tie, and a Stetson Gambler—one version of his performance costume of recent years, his unabashed yet half-ironic guise as the professional entertainer and itinerant rake. He was carrying a plastic bottle connected to what looked like a yellow gas mask. He pressed it to his mouth and inhaled deeply. “This is my ‘Blue Velvet’ device,” he said. The bottle contained a eucalyptus solution, to soothe his throat. He was fighting a cold that he feared he’d caught from his twin sons, who are about to turn four. The bug had prevented them from coming along with him to Sonoma. When people asked how he was, he replied, “I’m good, apart from not having the boys with me.”