About a dozen years ago, the Rough Guides and Lonely Planet series of travel books, rival bibles for the footloose and fancy-free, crossed a new frontier, venturing onto the Internet. But they found their road maps to the digital future hard to read. Guidebooks were soon overtaken online by Internet-era upstarts like TripAdvisor, which has built a business by drawing content from volunteer contributors and revenue from links to online reservations systems and advertising.

Now travel publishers are trying to catch up. They are moving more of their work onto the Internet and extending their content and brands into new areas like mobile services, in-flight entertainment systems and satellite navigation devices. Travel books themselves are getting a makeover, too.

And the recent acquisition by BBC Worldwide, the commercial arm of the British Broadcasting Corp., of a majority stake in Lonely Planet has prompted that publisher and its rivals to accelerate their search for new sources of revenue in the online world and elsewhere.

"We want to be in a position where, if the business suddenly collapses in five years, we have a plan - unlike the music industry," said Martin Dunford, publishing director of Rough Guides, which is part of the Penguin division of the media company Pearson, based in London.

So far, the digital media revolution has been much less turbulent for guidebook publishers than for record companies, which have fallen victim to rampant online copying. Sales of travel guides, while flat in some traditionally stalwart markets, like Britain, have been growing strongly in developing countries and in the United States - despite a weak dollar, which has made overseas trips more expensive for U.S. citizens. Travel publishers sold 14.8 million books in the United States last year, up 11 percent from two years earlier, according to Nielsen Bookscan.

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Still, guidebook companies may have missed an opportunity on the Internet, which presents them with moneymaking possibilities that have not generally been open to publishers of other kinds of books.

"Given how conducive the Internet is to what they do, they were probably a bit slow in developing this side of the business," said Alexander Burmaster, an analyst at Nielsen Online, which tracks Internet traffic.

TripAdvisor spotted the potential in tapping users' reviews of hotels, package trips and tourist attractions, and collecting a fee each time they click through to reserve a room, for instance, on a partner site. TripAdvisor supplements users' reviews with links to sites run by guidebook publishers like Frommer's. TripAdvisor, which is owned by Expedia, does not break out financial figures separately from its parent.

In reaching Internet audiences, TripAdvisor has clearly been a big success, placing third among travel-related Web sites worldwide, according to Nielsen Online. About 3.6 percent of users of travel Web sites visit TripAdvisor in an average month, placing it third behind Expedia and another booking service, Orbitz. Among guidebook sites, Lonely Planet ranks first, Nielsen says, but with an audience reach of only 0.3 percent.

While many travel publishers have had Web sites for a long time, some of them, along with booksellers, initially worried about cannibalizing sales of guidebooks. The easy availability of travel information online may indeed have cut into sales of guides to mainstream destinations, publishers say; Londoners traveling to Paris for the weekend are less likely than they used to be to buy an entire Lonely Planet guide to France.

But new book formats, aimed at niche interests and travelers taking short breaks on low-cost flights, for example, have made up for that shortfall.

Meanwhile, more and more guidebook content is being uploaded to the Internet, where it is often available free. Everything that appears in the Dorling Kindersley Eyewitness Top 10 guides, which feature only the highlights of a destination, is already available online, said Douglas Amrine, publisher of Dorling Kindersley travel books, another division of Penguin.

Alastair Sawday Publishing, a smaller travel publisher based in England, this past summer put all of its content, which consists mostly of hotel reviews, onto the Internet; previously, only 30 words of each review were available. Joe Green, who runs the Web site, said that the move was aimed at developing Internet advertising as a new source of revenue, to complement sales of the books and income from hotels that pay to be listed in them.

Lonely Planet plans to put all of its content onto the Internet within less than two years, said Judy Slatyer, chief executive of the company. Not all of that content will be free, though.