At 60, author and humorist David Sedaris is about to publish his 10th book, a collection of extracts from his long-running diaries. (Ingrid Christie/via Little, Brown)

I think I peaked in 1988, and it’s all downhill now. How awful, to decline this way. What makes young people young is that they see themselves going up, up, up. Not me, though. I’m old now. — David Sedaris, Jan. 13, 1992

The diary entry above was written when Sedaris was 35, 11 months before he ever appeared on National Public Radio. Two years before his first book, “Barrel Fever,” was published. Twenty-five years before this week’s release of his 10th book, which will add to the more than 10 million he has sold.

Today he is old — or at least, at 60, coming up on it. He has spent the past two years traversing the previous 40 to produce his latest work, “Theft by Finding: Diaries 1977-2002.”

It was a process that left him exhausted, exasperated and surprised at how little he has changed.

Take that 1992 entry, for example. Upon finding it, he reflected, “Well, that’s the first time I thought that. I think that all the time.”

Sedaris says this while making his way through New York’s Metropolitan Museum of Art, his ever-present Fitbit recording every step. Five days from the release of his new book, he is thinking something similar now: “The book landscape changes. Maybe people won’t buy the book. Maybe I’ve peaked.”

At first, he says that his advancing years allow him to greet this prospect with equanimity. “One thing I never realized about people’s decline is that a lot of it makes sense. You think, ‘Well, it’s time for a young person to have this opportunity.’ So you’re not envious and you think, ‘Oh, I had my time.’ ”

But then: “I think, ‘Oh, what if nobody comes on this book tour?’ Yeah, I’ll be disappointed by it and I’ll be embarrassed. . . .” he says, trailing off.

See? Not so different after all.

Sedaris started keeping a diary while hitchhiking in 1977, at age 20. “It was on the back of a place mat. At a roadside diner,” he says. “It was all very cliche. I had a beret on.”

He had no return address, so he couldn’t receive mail. But he could write to himself — about sleeping on a golf course, cooking soft-boiled eggs, finding dead birds under an interstate bridge. Journaling his life grew into a compulsion that continues to this day.

“Not doing it is not possible,” he says. “The world would spin off its axis.”

This urge to catalogue his observations has fueled his success as a writer and satirist. The early entries in the new book are heavy on drugs, money woes and conversations overheard during his nightly visits to the International House of Pancakes, where he could read uninterrupted for hours. Even then the wry voice that has made him nearly a household name is present, finding humor in both the mundane and the absurd.

In 1986 he captured an encounter with a young woman who approached him at a Chicago laundromat.

“What days do we eat meat?” she asked.

I thought it was a riddle at first. I mean, who’s the “we” here? I told her we eat meat whenever we want to, or can afford to.

“Can we eat meat three times a day?” she asked.

“Sure,” I said. “If we feel up to it.”

Sedaris devotees will love reading unfiltered accounts of his most familiar stories — his stint working as an elf at Macy’s, for example — and conversations with friends and family members who are like beloved characters in the universe he has created.

“This spring I am, if I’m not mistaken, in love,” he wrote in March 1991, after meeting Hugh Hamrick, his longtime boyfriend. He writes of his love for his mother, and of her death in 1991 — “I can’t believe this has happened.”

Beyond that, the entries contain precious little of Sedaris’s inner world. But they are a master class in bearing witness to the world around you. Just paying attention — something he thinks people do less and less these days.

“People staring at their phones miss things they might have seen,” he says. In the museum, they’re taking photos of themselves instead of ingesting the art they’ve purportedly come to see.

“That painting turned into a backdrop for the selfie,” he says of a woman documenting herself. “So it’s just about proving you went to a museum.”

If he sounds like a slightly grumpy old man, that’s fine. Sedaris has figured out certain advantages to aging. He thinks, for example, that this would be a fine time to rob someone. Because “once people have gray hair they all look alike.”

The police would ask, “What did he look like? ‘Well, he was old.’ That’s all they’ll say.”

In truth, Sedaris does get recognized, though usually not more than once or twice a day.

“I wouldn’t want any more,” he says. “Because you can’t really spy on people when they’re spying on you.”

Sedaris at the Metropolitan Museum of Art. (Ellen McCarthy/The Washington Post)

Sedaris is in New York for publication day and to start his book tour — he’ll do a reading at Washington’s Politics and Prose on Thursday. He stays at a hotel rather than with his famous sister, actress Amy Sedaris, because she has a rabbit. “And if you leave anything out, the rabbit chews it,” he explains. “It runs around at large, you know.”

Most of his time now is spent with Hamrick at their home in West Sussex, England. There he wakes up, writes in his diary, walks 15 to 20 miles picking up trash, writes some more and then, often, goes out to collect more trash. He has picked up so much litter that the town council named a garbage truck after him — the Pig Pen Sedaris — and he has been honored at Buckingham Palace.

The trash collection is a byproduct, though. The walking — and thinking and listening and absorbing — is the main thing.

“It’s not a choice that I’m making,” he says. It’s also just part of what’s required to be a writer — time spent alone. “You can’t let anybody get in your way,” he says. “You just can’t. ”

The hardest thing about editing more than eight million words of diary entries down to two 500-plus-page books — this is the first of two volumes — was living simultaneously with his past and present selves. “It was just a bit too much of me for those years,” Sedaris says.

But it was also a reminder of how much of his life turned out even better than he’d ever hoped.

“It’s just as good as you thought it was gonna be when things work out,” he says. “It’s just exactly as good as you think it is.”