He Sang ‘What a Fool Believes.’ But Michael McDonald Is in on the Joke.

The voice of Michael McDonald has been compared to velvet, silk and sandpaper, melted chocolate and last year, by a besotted 11-year-old girl, an angel. He has harmonized with the best in the business. But his latest duet might cause even the most Botoxed foreheads of Hollywood to furrow.

“How you like us so far?” joked Paul Reiser, the actor and comedian, from one corner of a squishy sofa in McDonald’s Santa Barbara, Calif., aerie on a recent Tuesday morning. He was there to talk about the singer’s memoir, which they wrote together and will be published by Dey Street Books on May 21.

In the other corner, emanating the equanimity that’s as beloved as his baritone, was the man whose 50-plus-year career has included backup vocals for Steely Dan, Elton John, El DeBarge, Toto, Bonnie Raitt and on and on — backup so extensive and distinctive it’s inspired playlists on Apple Music and Spotify. He was wearing a paisley-patterned shirt, black trousers and, as one might expect of an angel who must tread this cursed Earth, puffy Hoka sneakers.

McDonald, 72, has also spent decades in the spotlight, albeit sidlingly, often with his famous blue eyes shut. (“Singing is such an intimate act,” he explains in the book, “and like kissing, it does no real good to see what the other person is doing.”) He led the Doobie Brothers in various iterations with his gospel-inflected keyboard style; released nine solo studio albums traversing multiple genres and continues to make live appearances at venues from Coachella to the Carlyle.

Paul Reiser, left, with McDonald. The actor, known for “Mad About You,” said his musical collaborator is “very introspective.”Credit…Ariel Fisher for The New York Times

The book is titled “What a Fool Believes,” after the Grammy-winning hit McDonald wrote in 1978 with Kenny Loggins, though with some hesitation. “I thought, ‘Well, that’s just too obvious,’” he said. “I wanted it to be something clever and mind-provoking, and I couldn’t really think of anything because, you know, I have a problem provoking my own mind.”