As he prepares for the Sept. 20 release of his "Duets II" album, Tony Bennett promises "this will be the last one." But didn't he say that when he released the original "Duets: An American Classic" in 2006? "Well, I had no idea the first one was going to be that big," Bennett tells Billboard.com.

The first "Duets" debuted at No. 3 on the Billboard 200 and sold more than three million copies worldwide.

"The sales were so phenomenal (for 'Duets') that I was almost strangled by Sony Columbia... Actually, they said, 'Please do another one like that.' They were very nice about it. But I won't do another one."

"Duets II" pairs Bennett with another all-star cast of partners, ranging from icons (Aretha Franklin, Willie Nelson, Andrea Bocelli) to upstarts (Lady Gaga, Norah Jones, Josh Groban, John Mayer, Carrie Underwood) and those in between, including Natalie Cole, Mariah Carey, Sheryl Crow and others.

But even with that potent lineup, the album also has additional notoriety for Bennett's rendition of "Body and Soul" with the late Amy Winehouse.

He's now planning to release the song as the album's second single -- following "Don't Get Around Much Anymore" with Michael Buble -- in September, with proceeds likely earmarked for the foundation Winehouse's family is setting up in her name to help young people in need of medical or financial assistance.

"It's on film... and I think it will surprise everybody as to how well we ended up getting along," says Bennett, who recorded with Winehouse at EMI's Abbey Road studios in London.

"She was a little apprehensive about how to go about it, and I said to her, 'I may be wrong, but it sounds like you're influenced by Dinah Washington,' and that just blew her mind. She just said, 'Oh my God, you mean you can actually hear that? She's my idol!' And that relaxed her, and that's the record we ended up making."

Bennett says he had provided some counsel to Winehouse prior to the recording sessions, when she and her father, Mitch Winehouse, came to see him perform at London's Royal Albert Hall.

"I was convinced I would be able to help her and talk her out of... taking drugs," he notes, adding that the foundation is "a great way to turn something positive out of this."

Bennett acknowledges that the Lady Gaga duet on "The Lady is a Tramp" also seems out of left field but proclaims that "I never met a more talented person in my life... I think she's going to become as big as Elvis Presley."

Bennett says John Mayer, who joins him on "One For My Baby," is "a very talented guy and a great blues singer," and joining forces with Franklin for "How Do You Keep the Music Playing" was "just great. She's in top shape after her recent surgery -- looks great, sounds great. She knew exactly what to do."

Ultimately, Bennett says, all of his duet partners came to the project in top form. "I can't tell you how professional every one of them was, and how they worked and memorized and knew what they were going to do," he says. "And it shows up on the record, you know? You can hear them performing very well for us."

All of the sessions were filmed and will be released as both online content as well as a making-of documentary, while Bennett's son and manager Danny is also working on a feature film called "The Zen of Tony Bennett" that he says is "less a making-of and more Tony's philosophy, looking at a 60-year career in the context of this new project. "

"We captured all these wonderful conversations between (the singers) and Tony talking about music and art and family. It's going to be a very in-depth film."

Bennett, who celebrated his 85th birthday on Aug. 3, is preparing for some special shows in the coming weeks.

He'll play the Metropolitan Opera House in New York on Sept. 18 and the Staples Center in Los Angeles on Sept. 24, both of which are expected to feature guests from his pair of "Duets" albums. A large-scale, career-spanning box set, meanwhile, is due out this fall.