For more than 500 years the book has been a remarkably stable entity: a coherent string of connected words, printed on paper and bound between covers.

But in the age of the iPhone, Kindle and YouTube, the notion of the book is becoming increasingly elastic as publishers mash together text, video and Web features in a scramble to keep readers interested in an archaic form of entertainment.

On Thursday, for instance, Simon & Schuster, the publisher of Ernest Hemingway and Stephen King, is working with a multimedia partner to introduce four new releases called “;vooks,”; which intersperse videos throughout electronic text that can be read—and viewed—online or on an iPhone or iPod Touch.

And in early September Anthony E. Zuiker, creator of the television series “;CSI,”; released “;Level 26: Dark Origins,”; a novel—published on paper, as an e-book and in an audio version—in which readers are invited to log on to a Web site every five chapters to watch brief videos that flesh out the plot.

Some publishers say this kind of multimedia hybrid is necessary to lure modern readers who crave something different. But reading experts question whether fiddling with the parameters of books ultimately degrades the act of reading.

“;There is no question that these new media are going to be superb at engaging and interesting the reader,”; said Maryanne Wolf, a professor of child development at Tufts University and author of “;Proust and the Squid: The Story and Science of the Reading Brain.”; But, she added, “;can you any longer read Henry James or George Eliot? Do you have the patience?”;

The most obvious way technology has changed the literary world is with electronic books. Over the past year devices like Amazon's Kindle and Sony's Reader have gained in popularity. But the digital editions displayed on these devices remain largely faithful to the traditional idea of a book by using words—and occasional pictures—to tell a story or explain a subject.

The new hybrids add much more. In one of the Simon & Schuster vooks, a fitness and diet title, readers can click on videos that show them how to perform the exercises. A beauty book contains videos that demonstrate how to make homemade skin-care potions.

Not just how-tos are getting the cinematic work-up. Simon & Schuster is also releasing two digital novels combining text with videos a minute or 90 seconds long that supplement—and in some cases advance—the story line.

In “;Embassy,”; a short thriller about a kidnapping written by Richard Doetsch, a video snippet that resembles a newscast reveals that the victim is the mayor's daughter, replacing some of Doetsch's original text.

“;Everybody is trying to think about how books and information will best be put together in the 21st century,”; said Judith Curr, publisher of Atria Books, the Simon & Schuster imprint that is releasing the electronic editions in partnership with Vook, a multimedia company. She added: “;You can't just be linear anymore with your text.”;

In some cases, social-networking technologies enable conversations among readers that will influence how books are written.

The children's division of HarperCollins recently released the first in a young-adult mystery series called “;The Amanda Project,”; and has invited readers to discuss clues and characters on a Web site. As the series continues, some of the reader comments may be incorporated into minor characters or subplots.

While publishers will continue to release books written by authors working alone, Susan Katz, publisher of HarperCollins Children's Books, predicted that “;there is going to be a popular kind of literature where the author is seen as the leader of a large group and will pick and choose from these suggestions”; that readers contribute.

Bradley J. Inman, chief executive of Vook, said readers who viewed prototypes of “;The 90-Second Fitness Solution”; by Pete Cerqua or “;Return to Beauty”; by Narine Nikogosian “;intuitively saw the benefits of augmenting how-to books with video segments.”; Inman said readers then “;warmed to”; the fictional editions.

Jude Deveraux, a popular romance author who has written 36 straightforward text novels, said she loved experimenting with “;Promises,”; an exclusive vook set on a 19th-century South Carolina plantation in which the integrated videos add snippets of dialogue and atmosphere.

Deveraux said she envisioned new versions of books enhanced by music or even perfume. “;I'd like to use all the senses,”; she said.

Brian Tart, publisher of Dutton, an imprint of Penguin Group USA, which released “;Level 26,”; said he wanted the book's text to be able to stand on its own, but the culture demanded rethinking the format. “;Like everybody, you see people watching these three-minute YouTube videos and using social networks,”; Tart said. “;And there is an opportunity here to bring in more people who might have thought they were into the new media world.”;

Readers of “;Level 26,”; which Zuiker wrote with Duane Swierczynski, have had a mixed response to what the publisher is marketing as a “;digi-novel.”;

“;It really makes a story more real if you know what the characters look like,”; wrote Fred L. Gronvall, who gave the book—which he listened to in audio form—a top five-star rating in a review on Amazon.com. The videos, he wrote, “;add to the experience in a big way.”;

But another reviewer, posting as Rj Granados, wrote, “;Do you really think cheesy video vignettes will IMPROVE the book?”;

Some authors believe the new technologies can enrich books. For his history of street songs in 18th-century France, Robert Darnton, director of the Harvard University Library, will include links to recordings of the actual tunes.

But Darnton, author of “;The Case for Books: Past, Present and Future,”; warned that reading itself was changing, and not necessarily for the better. “;I think we can see enough already to worry about the loss of a certain kind of sustained reading,”; he said.

Doetsch, the author of “;Embassy,”; said the new editions should not replace the traditional book. He has written a forthcoming novel, “;The Thirteenth Hour,”; that he thinks is too long to lend itself to the video-enhanced format. The new editions, he said, are “;like dipping a novel into a cinematic pool and pulling it out and getting the best parts of each.”;

Some authors scoff at the idea of mixing the two mediums. “;As a novelist I would never ever”; allow videos to substitute for prose, said Walter Mosley, the author of “;Devil in a Blue Dress”; and other novels.

“;Reading is one of the few experiences we have outside of relationships in which our cognitive abilities grow,”; Mosley said. “;And our cognitive abilities actually go backwards when we're watching television or doing stuff on computers.”;