NEW YORK >> The best time to turn a pay phone into a lending library is early on a Sunday morning, said John H. Locke, an Upper West Side architectural designer who may be the world’s leading expert on the subject.

“There aren’t a lot of people out,” he said. “You can just go down, find a good booth, carry it out, latch it in. It takes seconds. And then just fill it up with books and let’s wait and see what happens.”

Last winter, Locke designed a lightweight set of bookshelves to fit inside the common Titan brand of New York City pay phone kiosks. A fabricator in Brooklyn cuts the shelves, which Locke paints and assembles in his apartment. So far he has carried out four installations, most recently at Amsterdam Avenue and West 87th Street just before 8 a.m. on a Sunday last month.

As several sleepy-eyed patrons of a 24-hour deli looked on in confusion, Locke snapped a lime green bookcase into place, stocking it with children’s books and paperback novels. Hooks on the unit allow Locke to install it without hardware, and the entire process took less than five minutes. He had barely rounded the corner before a man who had been standing outside the deli began browsing through titles, choosing “The Shining” by Stephen King, tucking it under his arm and heading home.

What happens to the installations after the first few minutes is a bit of a mystery to Locke. He checks on them periodically, he said, until they disappear — after a few days or a few weeks. Which is fine with him.

“It’s a spontaneous thing that just erupts at certain locations,” he said. “People like it, people are inspired by it, but then it disappears again.”

The libraries have endured long enough to attract their share of fans. Publishing houses, bookstores and neighbors have approached Locke to donate books for future installations. The project is currently being featured in Spontaneous Interventions, the United States’ contribution to the International Venice Architecture Biennale, an architecture show.

If any disused fixture of city streets cried out for repurposing, it would seem to be the pay phone. The city’s Department of Information Technology and Telecommunications acknowledges as much. In July, the department began soliciting ideas about what to do with the city’s remaining 13,000 sidewalk pay phones once the current contracts expire in 2014.

Not that they are completely obsolete. The city cites federal data showing that the number of pay phones nationwide declined to 872,000 in 2007, from 2.1 million in 1999. But the average pay phone here was used to place six calls a day in 2011, not including emergency calls, according to the city. And in a single week in December, 8,264 calls were made to 911 from sidewalk pay phones.

Perhaps more important to the city, pay phones brought in $18 million in revenue in the last fiscal year. Of that, only about $1 million came from callers’ quarters; the rest came from advertisements displayed on the side of the phones’ cabinets. Since the agency would be loath to give up that money, it is considering the suggestions that it turn phone booths into touch-screen neighborhood maps; convert them into charging stations for mobile devices or electric cars; or use them as dispensers for hand sanitizer.

“Maybe not the idea we’ll ultimately go with,” Brett Sikoff, the department’s director of telecom franchises and special projects, said of that last suggestion, “but it certainly shows the creativity of the public.”

The city is also engaged in a pilot project to use pay phones as Wi-Fi hot spots. Eleven pay phones, including ones in every borough but the Bronx, have been providing free Wi-Fi since July. About 2,000 people logged on to the networks in August, according to the city. Users stayed connected for an average of 38 minutes.

Locke, who has an aversion to outdoor advertising, said he wanted nothing to do with the city’s initiative. He does post the plans for his shelves on his website, in the hope that people will install their own versions in their own neighborhoods.

On Amsterdam Avenue, Locke’s latest library met a mixed reaction.

Everything started off well. Robert Davis, the man who took “The Shining,” raved about the idea, though he was under the mistaken impression that it was part of a citywide effort by the Bloomberg administration to improve literacy.

“I love reading,” he said. “Shame about the Internet, though. People should never give up on books.”

Within minutes of Davis’ departure, another man arrived, wearing suspenders over a white T-shirt. He leafed through an old hardcover but left empty-handed. A young girl in a yellow New York City Housing Authority shirt tried in vain to convince her mother to let her take a picture book before being dragged away.

But in large part, the bookshelf served to highlight New Yorkers’ ability to ignore anything in their paths. Locke said he was impressed that for each person who seemed to notice the bookshelf, at least a dozen walked by completely oblivious.

At times this seemed to be a determined choice. One young woman refused to break stride even when her companion stopped and pointed.

“That is awesome,” he said with enthusiasm.

“OK, I don’t think I can stop for that,” she said, without turning her head.

After Locke had headed back home to have breakfast with his fiancee, four men arrived who were very interested in the project. They spoke in hushed tones and glanced up and down the block. One pretended to use the phone peeking out of the bookshelf. Then, the coast clear, they emptied the shelves of their contents. Everything went into their blue plastic bags, from “Leverage Leadership: A Practical Guide to Building Exceptional Schools” by Paul Bambrick-Santoyo to “The Short Stories of F. Scott Fitzgerald.” The men shouldered the bags and went their separate ways.

Behind them they left an empty bookcase, now good for nothing — unless someone was actually looking to make a phone call.