"On the Road" sits alone on a seat aboard the Coaster. "The Grapes of Wrath" dangles from a tree in a plastic bag. "The Catcher in the Rye" is discovered in the bread section of a grocery store.

Where do they come from? Absent-minded readers? Literate pranksters with a taste for the surreal?

A look inside the front cover likely will solve the mystery. Chances are, each book contains a label about BookCrossing and is participating in an international exercise aimed at sharing literature around the world.

"It's exceeded our expectations," said Ron Hornbaker, who founded BookCrossing in 2001 and operates it from his home in Kansas City, Mo. "The first year we got 1,000 members. The second year, 100,000 members. Now that's sort of trailing off, but word of mouth and people finding a book is what's responsible for 400 new members a day."

More than 235,000 people in more than 150 countries participate in BookCrossing, which combines the childhood thrill of releasing a message in a bottle with the global connection possible through the World Wide Web.

Participants register a book at www.BookCrossing.com with an identification number, which then is written on a label inside the cover. Then the book is left in a public place. The label asks whoever finds the book to visit the Web site and make a journal entry about the find, and then to release the book "back into the wild" when the finder is finished reading it.

The site sells stamps and labels to make the labeling process easier, but you can also download free labels.

Ann Alcocer, 45, is an Oceanside resident who has released 90 books.

"I've left them at the train station, I've left them at the zoo," said Alcocer, who calls herself MissTree on the Web site. "Any public place."

Alcocer said she has participated in "theme releases" with other BookCrossing members, such as the time she and others hung books from plastic bags outside the North County Times' Oceanside office last August.

"I guess I strongly believe in literacy," she said about her motivation. "And it's just really a fun thing to do. You might make somebody's day."

Alcocer has never found a BookCrossing book by accident, but she has tracked some down through the "go-hunting" section of the Web site, which tells members where a book has been placed. She also has given and received books at meetings with other members.

"I love to release books I've really enjoyed," she said. "Sometimes I'll get extra copies. Sometimes I'll go to library book sales and get books that are really inexpensive."

Alcocer has released some of her favorite books, including "Little Women" and novels by Jane Austen.

Hornbaker said between 20 and 25 percent of released books are reported found by people who go to the Web site and write about the discovery.

"That is the biggest drawback to the whole thing," Hornbaker acknowledged. "Some people feel like they're going to lose track of these books, but our core membership, they keep the faith. We've had people go over two years before they get their first journal entry."

Larry Gundersen of Carlsbad, 64, grew frustrated at the book-exchange project because of the lack of responses he received.

"The response rate has been so disappointing, I gave up on the concept," he said. "I just don't get any response back. I did a flurry of them a year and a half ago. I was putting out one or two a day for a couple of months."

Gundersen, a biochemist and director of a master's degree program at San Diego State University, registered books by authors ranging from Ayn Rand to Erma Bombeck.

"I liberated in excess of 50 books," he said. "I put labels on them, left them on a park bench outside the office of my university. They all quickly disappeared."

The response was especially disappointing to Gundersen, considering the success he has had with Where's George? the past three years.

In that project, participants stamp a dollar bill with instructions on how to track it through the Web site WheresGeorge.com. Gundersen said he has received 930 reports about dollars he's released, which have found their way to 37 states, Guam and Manitoba.

Hornbaker said Where's George? was one of the inspirations of BookCrossing.

While a member has no control over who may pick up a released book, Hornbaker said people can improve their odds of getting responses by releasing books they think people will actually read rather than just using the project as a way to clear off a crowded bookshelf.

"If you stumble upon a book in the wild and it's appealing to you, you'll be far more likely to pick it up and read it," he said.

Hornbaker suspects that people generally leave books that mean something to them or contain a message they want to share.

"We get a lot of 'Fahrenheit 451' or 'Catcher in the Rye,'" he said, adding that released books tend to be popular ones that often are on a current best-sellers list.

"Typically, BookCrossers want others to experience a love of literature like they do," he said. "I think the best thing about the BookCrossing personality is they're not only literate, but generous."

Another thing most members have in common is gender. Hornbaker said about 85 percent are women, with the largest age group between 25 to 45 years old.

Mary Zornado of San Marcos, 39, said she likes the relay section of the site, where one member offers to mail a book, another accepts it and agrees to mail another book of the same theme or genre in its place.

While that is an efficient way of getting and giving books, Zornado said there's a special feeling in releasing books into the wild. She learned about the project by discovering a labeled book at a library sale.

"I remember thinking, 'Oh my gosh, somebody actually found it and enjoyed it,'" she said about receiving her first response from a person who happened upon one of her books. "That's what the payoff is. Someone who responds to this random act of kindness. That's what makes it fun."

Zornado said she's released about 50 books. Some are left at coffee shops and hair salons, and she's left books on baby care at doctor's offices for expectant mothers to find.

Maggie Schulte of Carlsbad also said she gets a thrill out of releasing books to the wild.

"It's kind of like playing 'Hide and Seek' without the other person knowing they're in the game," she said.

Schulte has given away 233 books, with 85 percent of those wild-releases and the rest gifts to other members. She also remembers the first response she received.

"I was very excited that someone had picked the book and was going to read it and re-release it," she said.

Kathryn Allen, 62, said she likes to leave books near her workplace in downtown San Diego and on benches at the Oceana community in Oceanside.

"It's a lot of fun," she said. "I have read books and discovered books that I would not have read. It's really broadened my reading habits."

Allen, an Oceana resident, said she sometimes has rushed out to look for a book after receiving e-mail alerts that somebody had left one in downtown San Diego, but so far hasn't found one. She also has never received a response back about any of the books she has released, although all but one have been picked up.

The one exception, she said, is "The Intelligence of Dogs," which she described as an old book about communicating with and caring for dogs.

"It's been there four months," she said. "I left it with a big sticker on it that said 'Free, this is a BookCrossing book.'"

She left the book at the dog park in Oceana, which she assumed would be a natural place to be discovered.

If you're an Oceana resident interested in a BookCrossing find, Allen said "The Intelligence of Dogs" was still there the last time she looked.

Contact staff writer Gary Warth at gwarth@nctimes.com or (760) 740-5410.