David Putnam, a D.C. legal consultant, takes voyeuristic pleasure in watching strangers find books he leaves on coffee shop tables. He'll plant a book, buy a cuppa and settle in to watch. On the weekends, for kicks, Putnam and his girlfriend will release a book together. The last book he planted in a coffee shop was about the CIA.

"It's got this camouflage cover," he says. "You see people glance at it and look away as if they're afraid of it." An avid reader (about a book a week, he says), the 28-year-old Putnam first registers each book at www.bookcrossing.com. He labels them with an online registration number and the address of the Web site, and the finders can log on to give their impressions of what they've read.

The Book Crossing site, started by 36-year-old software developer Ron Hornbaker in April 2001, now boasts more than 74,000 members worldwide. Hornbaker's aim was to create a community of readers interested in tracking their books' journeys and participating in a dialogue on the literature.

Readers from more than 80 nations have registered books before "releasing them into the wild." Although he's partial to coffee shops, Putnam also enjoys planting books at the Albert Einstein Memorial on Constitution Avenue; he recently nestled "Finite and Infinite Games," by James P. Carse, in the nook formed by the physicist's right hand. A check of Mr. Einstein a few days after Putnam's release revealed that the treasure had been "caught" -- the Book Crossing term for finding a book in public. Whoever took "Finite and Infinite Games" has yet to reveal himself on the Book Crossing site.

When Putnam first left a book at the memorial, a child in a school group from Oregon picked it up. "You can imagine somebody -- in this case a bored tourist at the Einstein Memorial -- saying, 'Hey, what's that book there,' " he says. The student logged on to Book Crossing to report that the book, a collection from the satirical newspaper the Onion, livened up the group's time on the bus.

No one has ever registered a catch from one of Putnam's other hiding spots -- a park bench near his office. Could the books have been swept into the trash at the end of the day? The books that "Book Crossers" leave all over the world -- on buses, subways and airplanes, for example -- could conceivably just inconvenience whoever has to clean up our public places. "I'm sure that happens sometimes," Putnam says.

Andrea Bacino, for one, doesn't think it's a problem. She left John Steinbeck's "The Grapes of Wrath" in the Gate D waiting area in Union Station last month. No one has logged on yet to tell of its fate. "There's a taboo against throwing away books," says the high school English teacher. "People don't throw out books; at least, that's what I hope." Bacino, 22, is not accustomed to letting her books go that easily. She left "The Grapes of Wrath" to its fate only because she has another copy.

The site thrives because when people finish one book, they want another. Putnam points out that he doesn't have "a country estate with a huge library." Giving away used books, he says, "meshes with city life."

Putnam has not yet come across any books in the wild. With fewer than 1,000 members, he says, "it's not yet to the point where D.C. is choked with awesome books."

Hornbaker, who started the site as an outreach project for his software company Humankind Systems, says that at first only about 10 percent of the released books were registered as caught. As the site has grown and Book Crossers have learned to make better book labels, the catch rate has increased to more than 25 percent.

In Book Crossing's first 10 months, Hornbaker says, the site grew by about 100 members a month. When it began getting media attention last spring, membership exploded, and the site now logs about 350 new members a day. "The neat thing is we're only nine months into this, if you look at our main growth," Hornbaker says. "Ten years from now, looking on to the paths these books have taken is going to be fun." He encourages members to make Book Crossing their "virtual library," registering every book they have. "Think of your grandkids," he says. "They can go to the site and see what their grandfather thought 70 years ago."

When he started the site, Hornbaker, who lives in Kansas City with his wife and daughter, worried that publishers would complain or try to interfere. "I didn't want to be known as the Napster of books," he says. But since books cannot be easily reproduced like music, he ultimately decided that Book Crossing would not become a target. In fact, publishers have done promotions using Book Crossing. For a PBS movie based on Tony Hillerman's "Skinwalkers," for example, the marketing agency RPMC labeled and released 1,000 copies of the book. "It's a way of bonding with readers and generating word of mouth," says RPMC Senior Vice President Craig McAnsh.

Hornbaker's ultimate goal for the site is for users to discuss the books they are reading, not just report that they found the book. "That urge people have when they finish a book to talk about it" drives the site, he says. Putnam says he releases only books he likes, so he can see what other people think of them. For example, a Book Crosser who found one of Putnam's recent releases "totally contradicted" Putnam's observations on its bookcrossing.com page. Now Putnam will log back on to that page with a rebuttal. Thus, the literary dialogue begins.

Bacino sat in the Union Station food court recently, her remaining copy of "The Grapes of Wrath" prominently displayed, hoping for some of that give and take. Some Book Crossers had arranged to meet in person through another site, www.meetup.com, which facilitates gatherings among members of virtual communities. "When people have books in common, you always have something to talk about," Bacino says. Book Crossers all over the world get together on the second Tuesday of every month via meetup.com, with Italians holding the largest gatherings. Italy places third for site members, behind the United States and Canada.

Sadly, only Bacino -- who lives in Seattle but was visiting Washington for the holidays -- came to D.C.'s scheduled gathering, for which at least five people had committed. More than 700 people from the Washington area have registered on Book Crossing, and several dozen in turn have registered on meetup.com. Perhaps braving the cold of December to get to Union Station just didn't compare to curling up with a good book.

Ron Hornbaker's Book Crossing Web site has attracted more than 74,000 members worldwide.