Textbooks are gaining, though, as publishers take advantage of the popularity of tablets like the Kindle and iPad, expanding their catalogues and offering products like rental digital books that expire after a semester or two. The potential for digital growth is leading publishers to experiment with products that stretch the boundaries of traditional textbooks, slowly turning away from static text and images toward a multimedia, intuitive approach, publishers say. “Textbooks as e-books ought to be seen as a stepping stone to the future,” said Mark Majurey of Taylor & Francis, a textbook publisher in Britain. Digital textbooks are any books that can be downloaded to an e-reader or computer or those that can be read online using a Web browser. While no one keeps precise numbers of digital textbook sales globally, a number of companies have seen similar growth patterns and nearly identical market share. According to the Student Monitor, a private student market research company based in New Jersey, about 5 percent of all textbooks acquired in the autumn in the United States were digital textbooks. That is more than double the 2.1 percent of the spring semester. Simba Information, a research company specializing in publishing, estimates that electronic textbooks will generate $267.3 million this year in sales in the United States. That is a rise of 44.3 percent over last year. The American Association of Publishers estimates that the college textbooks industry generated a total of $4.58 billion in sales last year. Kathy Micky, a senior analyst at Simba, said digital textbooks were expected “to be the growth driver for the industry in the future.” Her company estimates that by 2013, digital textbooks will make up 11 percent of the textbook market revenue. Though some textbook publishers made some of their textbooks available in digital formats a decade ago, it is only recently that the market has picked up. Responding to the new demand, many academic publishers have made almost everything they sell available in electronic format. “All of our books are available as digital,” said Bruce Spatz, head of digital development at John Wiley & Sons, a major academic publisher. Early textbook digitalization attempts were also focused more on specialized textbooks. But now publishers are looking at wider markets, focusing on broad subjects and students just now entering college. “In general where the most money is made is in the introductory market,” said Ms. Mickey, noting that publishers were now focusing on supplying textbooks for first-year, introductory and core subjects — for courses taught to many students. Other entities expanding into the market include Chegg, a major paper textbook rental company in the United States, which started offering digital, downloadable books this year. It announced in August that it planned to expand to become what it called one of the most comprehensive electronic textbook retailers in the United States. CourseSmart, a partnership of five major U.S. textbook publishers that offers some 90 percent of textbooks used at North American universities online, announced its intention to expand overseas this year. Perhaps the biggest change could come from the rise of electronic rentals. Digital textbooks can be made to expire — often between six and 18 months after the initial purchase — which means they cannot be resold like traditional books. Most digital textbooks also only license the first owner, and sophisticated software ensures that copies cannot be passed around. These measures help ensure that prices for digital textbooks stay well bellow the cost of the paper versions, publishers say, even though those who print traditional paper books might take issue with that. “It might be a problem for the printers, but it isn’t a problem for the publishers,” said Bruce Hildebrand, who is in charge of the higher education sector at the Association of American Publishers. Early this summer, Amazon announced that it was partnering with three major textbook companies to offer rentals of digital textbooks for even shorter terms.