Kate Lambert recalls using her library card just once or twice throughout her childhood. Now, she uses it several times a month.

The lure? Electronic books she can download to her laptop. Beginning earlier this year, Lambert, a 19-year-old community college student in New Port Richey, Fla., borrowed volumes in the “;Hitchhiker's Guide to the Galaxy”; series, “;The Lovely Bones”; by Alice Sebold and a vampire novel by Laurell K. Hamilton, without ever visiting an actual branch.

“;I can just go online and type my library card number in and look through all the books that they have,”; said Lambert, who usually downloads from the comfort of her bedroom. And, she added, “;It's all for free.”;

Eager to attract digitally savvy patrons and capitalize on the growing popularity of electronic readers, public libraries across the country are expanding collections of books that reside on servers rather than shelves.

The idea is to capture borrowers who might not otherwise use the library, as well as to give existing customers the opportunity to try new formats.

“;People still think of libraries as old dusty books on shelves, and it's a perception we're always trying to fight,”; said Michael Colford, the director of information technology at the Boston Public Library. “;If we don't provide this material for them, they are just going to stop using the library altogether.”;

About 5,400 public libraries now offer e-books, as well as digitally downloadable audio books. The collections are still tiny compared with print troves. The New York Public Library, for example, has about 18,300 e-book titles, compared with 860,500 in circulating print titles, and purchases of digital books represent less than 1 percent of the library's overall acquisition budget.

But circulation is expanding quickly. The number of checkouts has grown to more than 1 million so far this year from 607,275 in all of 2007, according to OverDrive, a large provider of e-books to public libraries. NetLibrary, another provider of e-books to about 5,000 public libraries and a division of OCLC, a nonprofit library service organization, has seen circulation of e-books and digital audio books rise 21 percent over the past year.

Together with the Google books settlement—which the parties are currently modifying to satisfy the objections of the Department of Justice and others—the expansion of e-books into libraries heralds a future in which more reading will be done digitally.

“;As young people become used to reading virtually everything online,”; said Paul LeClerc, president of the New York Public Library, “;that is going to propel a change in terms of readership of e-books rather than readership of physical books.”;

For now, the expansion will be slowed in part because, with few exceptions, e-books in public libraries cannot be read on Amazon's Kindle, currently the best-selling electronic reader, or on Apple's iPhone, which has rapidly become a popular device for reading e-books. Most library editions are compatible with the Sony Reader, computers and a handful of other mobile devices.

The bulk of digital books in libraries are treated like printed ones: Only one borrower can check out an e-book at a time, and for popular titles, patrons must wait in line just as they do for physical books. After two to three weeks, the e-book automatically expires from a reader's account.

But some publishers worry that the convenience of borrowing books electronically could ultimately cut into sales of print editions.

“;I don't have to get in my car, go to the library, look at the book, check it out,”; said John Sargent, the chief executive of Macmillan, the publishing giant that owns imprints including Farrar, Straus & Giroux and St. Martin's. “;Instead, I'm sitting in the comfort of my living room and can say, 'Oh, that looks interesting' and download it.”;

As digital collections grow, Sargent said he feared a world in which “;pretty soon you're not paying for anything.”; In part because of such concerns, Macmillan does not allow its e-books to be offered in public libraries. The company publishes authors like Janet Evanovich, Augusten Burroughs and Jeffrey Eugenides.

Simon & Schuster, whose authors include Stephen King and Bob Woodward, has also refrained from distributing its e-books to public libraries. “;We have not found a business model that works for us and our authors,”; said Adam Rothberg, a spokesman.

For now, the advent of e-book borrowing has not threatened physical libraries by siphoning away visitors, because the recession has driven so many new users seeking free resources through library doors. And in some cases, few library patrons seem to know that e-book collections even exist.

In the Brooklyn Public Library system recently, eight people were waiting for three digital copies of “;The Lost Symbol,”; Dan Brown's follow-up to “;The Da Vinci Code,”; while 715 people were waiting for 526 print copies.

Some librarians suggest that because digital books never wear out, take no shelf space and can be read by multiple people at the same time, the business model should be different from the model for print.

Pam Sandlian Smith, library director of the Rangeview Library District, which serves a suburban community north of Denver, said she would prefer to buy one copy of a digital book and pay a nominal licensing fee each time a patron downloaded it.

“;It would help us with physical space, and we would be able to serve so many more people,”; Smith said.

Publishers, inevitably, are nervous about allowing too much of their intellectual property to be offered free. Brian Murray, the chief executive of HarperCollins Publishers Worldwide, said Smith's proposal was “;not a sustainable model for publishers or authors.”;

Some librarians object to the current pricing model because they often pay more for e-books than consumers who buy them on Amazon or in Sony's online store. Publishers generally charge the same price for e-books as they do for print editions, but online retailers subsidize the sale price of best-sellers by marking them down to $9.99.

“;'The Lost Symbol' is $9.99 on the Sony Reader book page, and I just paid $29.99 for that for the library,”; said Robin Bradford, the collection development librarian at the Indianapolis-Marion County Public Library. Bradford said she would consider buying additional digital copies if the price were lower. But “;to buy nonphysical copies at the same price,”; she said, “;I just won't do it.”;

Academic publishers have been more willing to experiment with subscription models, inviting libraries to pay an annual fee for unlimited access to certain books. Scholastic Inc., the children's book publisher, also offers library subscriptions to BookFlix, a collection of picture books that children can read online.

Steve Potash, the chief executive of OverDrive, said publishers should regard library e-books as a form of marketing. Many people who browse a library's online catalog end up buying the books, he said, although he could provide no evidence of that.

Some publishers agree that library e-books, like print versions, can attract new customers. “;We've always strongly believed that there is a conversion point where they do start to buy their own,”; said Malle Vallik, the director of digital content at Harlequin Enterprises, the romance publisher.

In libraries, readers are attracted to free material. Nancy Gobel, a dental hygienist who already downloads digital audio books from her library in Indianapolis, said she currently buys print books. But she is considering purchasing an electronic reader so she can borrow them for free. “;I would still continue to buy, but I would download as much as I can,”; she said. In many cases, she said, buying “;doesn't make sense.”;