Earlier this year Rod Stewart vowed putting the finishing touches on Thanks for the Memory ... The Great American Songbook Volume IV, he decided to leave himself some wiggle room, saying, "I would hate not to be able to do another one."

Stewart's first three easygoing collections of songs written by the likes of the Gershwins, Jerome Kern, Rodgers and Hart, Hoagy Carmichael, and Frank Loesser have all gone multiplatinum; the third volume debuted at No. 1 on the Billboard Top 200 chart in 2004, giving him his first No. 1 album since Blondes Have More Fun, in 1978. Volume III also won him a Grammy, after 14 hard-luck nominations. "I must admit my kids were more pleased than I was," he says. "I was under the impression I was doing all right without it."

The new album, out this October from Clive Davis's J Records, departs only slightly from the formula, with two rhythm-and-blues classics included among the Tin Pan Alley warhorses. Those songs will ease Stewart's undergarment-tossing fans into what may be his next project. "I think what we're going to do for the next album is The Great American Soul Book," he says. At press time Stewart was awaiting word from a possible duet partner on one of Volume IV's R&B tracks, Sam Cooke's "You Send Me." "It's supposed to have Christina Aguilera on it, but she hasn't done it thus far, so I'm keeping me fingers crossed," Stewart says in a Cockney lilt that 25 years of residing in Los Angeles has left unchanged. Aguilera is one of his favorites among the singing stars 30 and 40 years his junior. "She's superb," says Stewart, 60. "Nothing wrong with her. She's nearly as good as me."

"Mick's not a bad singer—but he's not as good as me. Ha-ha!"

Stewart has regularly assumed new guises, most notably when he went disco in 1978 with "Do Ya Think I'm Sexy?" While his contemporaries the Rolling Stones rake in cash on the road, Stewart is making a bigger dent in the charts these days. Is he happy to outsell his old mates (among them Ron Wood, Stewart's former colleague in the Faces)? "Nooo," he says with a raspy laugh. "They're survivors and still the best rock 'n' roll band, in my opinion. Mick's not a bad singer—but he's not as good as me. Ha-ha! Woodie keeps plaguing me—'When are you gonna see the band?' I said, 'I saw them in 1963; I'm not gonna see them again.' But I am going to try to see them this time."

With five of his six children living nearby or in his house, and a wedding set for next spring with 34-year-old former model Penny Lancaster (who's pregnant), Stewart has only one regret about getting older: a bum knee that keeps him off the soccer field. "Just in the middle of trying to rebuild it and see if I can give it one more heroic try," he says. "But I've had a good run on it, 50-odd years of playing, so I can't complain." Adopting the solemnity of a eulogist, he adds, "It's been a good knee."