Here's the idea: Take a book you've read, register it at the BookCrossing site, slap a special identifying label inside the cover, and leave the book in a public place.
When someone finds it and logs on to the Web site using the book's BookCrossing ID number, you get notified by e-mail.
Smith, a Mount Airy resident, has left 37 books (among them C.S. Lewis' The Screwtape Letters and Oliver Sacks' Awakenings) at area bus stops, train stations and bars.
Last week, he left Reinventing Comics by Scott McCloud on a bench in Chestnut Hill. It was picked up by Gwenne Sulby, who said she would take the book home and register it on the Web site.
"I think this is great," Sulby said. "I might try it."
Smith hasn't heard from the finders of any of the books he's left. But that doesn't bother him.
"I have so many books," said Smith, 45, who runs a financial services firm. "I don't want to sell them. I want someone to read them. Doing this makes me feel good about these books I care about."
BookCrossing's founder, Ron Hornbaker, the chief executive officer of a Kansas City, Mo., software development company, said he got the idea from Web sites such as wheresgeorge.com, which tracks the circulation of dollar bills, and phototag.org, which releases disposable cameras with instructions to take one shot and pass them on. (The results are published on the site when the cameras are returned.)
"It harkens back to sending out messages in a bottle as a child," said Hornbaker, who offers free membership and helps support the site (Motto: "Read and Release") by selling T-shirts, hats and tote-bags.
"Besides helping readers put their books out there for other like-minded people, BookCrossing allows them to connect to a virtual community," he said. "I get e-mails every day from people who say, 'We've been doing that all our lives - leaving books for others. Thanks for connecting us to this.' "
The year-old site has 20,000 members who have registered more than 46,000 books, mostly in the United States. Since a National Public Radio feature in May, BookCrossing has been getting 200 new members a day, Hornbaker said.
One Philadelphia BookCrossing member, who uses the screen name "Bodacious" on the site, explains her attraction this way: "In my childhood, a found book would have been a treasure beyond all hope. So now, to feed that child of memory, I get to leave books for others to find."
Kristal, an assistant district attorney who lives in Fairmount, said she'd been in the habit of leaving already-read books in her apartment building's community room. "I love the idea of being able to track where they go," she said.
Besides that copy of The Mammoth Hunters deposited in Rittenhouse Square, she's left Henrik Ibsen's play Ghosts in a bus shelter at 19th and Arch Streets and released Myth Directions, by sci-fi/fantasy writer Robert Asprin, in the Reading Terminal Market.
But passing books on isn't always easy. It took Mahon, a Temple University student who lives in Cherry Hill, two tries to unload a copy of Tuesdays With Morrie, one of her favorite books.
On her first attempt, a good Samaritan retrieved it and returned it to Mahon when she got off the train.
"When I tried to explain it to her, she said, 'Are you serious? You really leave books around for other people?' " Mahon said.
Smith said he'd twice been stymied by the owners of a Mount Airy bar who'd held on to the books he'd left behind. "People are mystified," he said. "When I came back in, they said, 'Are these your books?' "
It seems they hadn't looked inside the cover at the label alerting finders to the BookCrossing mission.
Proper release technique is a subject that's exhaustively debated on the site's e-mail forum. One idea to ensure a successful release: Affix a yellow Post-it note on the cover that says "Free Book! Look Inside!"
As for release locales, BookCrossers will try just about anyplace - the top of an ATM machine, inside a newspaper vending box, even the refrigerator case at a grocery store.
Earlier this month, according to a feature on the site that lists where books are being released, a Cambridge, Mass., BookCrossing member left tomes about the Ice Age and Internet culture on a tram car on the Wildwood boardwalk.
Robin Payton, 40, a Saint Peters, Mo., homemaker, holds the distinction of releasing more books than any other BookCrossing member. That's 441 to date, ranging from romance novels to Henry James.
She keeps a shelf of pre-labeled books next to the front door to ensure that she never goes out without one, and she has begun a project to release a book in every state.
To date, just 39 of Payton's releases have been "successful" - meaning she's heard back from a finder.
Hearing about the travels of her books, though, isn't what motivates her.
"The concept of filling the world with books appeals to me because books have been very important in my life," Payton said. "I enjoy thinking that one of my books might be the trigger for someone else's love of reading."
Contact Eils Lotozo at email@example.com or 215-854-5610.