But in the past year, he's finished 10 books, including Dan Brown's 'Inferno,' Walter Isaacson's biography of Steve Jobs and George R.R. Martin's fantasy series 'A Song of Ice and Fire.' He listens to audio books in snippets throughout the day on his iPhone during his morning workout, on his 20-minute commute to work, and while he's cooking dinner or cleaning up. Before he falls asleep, he switches to an e-book of the same story on his Kindle, and starts reading right where the narrator left off.

'I fly through a lot more books than I used to,' Mr. Wilbur said.

The digital revolution may have dealt a heavy blow to print, but it is boosting literacy in other unexpected ways by fueling the explosive growth of audio books.

Once a static niche for aficionados renting clunky cassettes or CDs for their commutes, audio books have gone mass-market. Sales have jumped by double digits in recent years. Shifts in digital technology have broadened the pool of potential listeners to include anyone with a smartphone.

At the same time, publishers are investing six-figure sums in splashy productions with dozens of narrators. Using the Netflix model, some audio book producers have even started experimenting with original works written exclusively as audio productions, ranging from full-cast dramatizations in the style of old school radio plays, complete with music and sound effects, to young adult novels, thrillers and multipart science fiction epics.

'It's one of the few times in history that technology has reinvigorated an art form rather than crushing it,' said Max Brooks, author of the zombie novel 'World War Z,' which was released in May ahead of the Brad Pitt movie in an elaborate new audio edition with 40 cast members, including Alan Alda, John Turturro, and Martin Scorsese. It sold 60,000 CDs and digital-audio copies. 'Now, because there is such demand and the production value is so inexpensive, it opens the door for more creative storytelling.' he said.

Digital innovation isn't just changing the way audio books are created, packaged and sold. It's starting to reshape the way readers consume literature, creating a new breed of literary omnivores who see narrated books and text as interchangeable. Last year, the audio book producer and retailer Audible unveiled a long-awaited syncing feature that allows book lovers to switch seamlessly between an e-book and a digital audio book, picking up the story at precisely the same sentence.

So far, Audible, which is owned by Amazon, has paired some 26,000 ebooks with professional narrations. The company is adding more than 1,000 titles a month and aims to eventually bring the number to close to 100,000.

'We're moving toward a media-agnostic consumer who doesn't think of the difference between textual and visual and auditory experience,' says Don Katz, Audible's founder and CEO. 'It's the story, and it is there for you in the way you want it.'

Audio books have ballooned into a $1.2 billion industry, up from $480 million in retail sales in 1997. Unit sales of downloaded audio books grew by nearly 30% in 2011 compared with 2010, according to the Audio Publishers Association. Now they can be downloaded onto smartphones with the tap of a finger, often for the price of an e-book.

Recorded books date back to the 1930s, when the Library of Congress created a 'talking books' program for the blind. The proliferation of the cassette deck in autos boosted their popularity in the 1980s. CDs followed, but sales plateaued as audio books failed to keep pace with other forms of digital entertainment.

These narrated books typically cost $50 or more ($100 for Stephen King's lengthy 'The Stand'), so devotees borrowed them from the library or rented them from mail-order services like Books on Tape, which still exists as a unit of Random House and emphasizes unabridged titles for libraries and schools.

Prices are much lower now, as production costs have plunged and demand has risen. Audio books can be bundled with an e-book for just a few dollars, downloaded as part of a monthly subscription plan or bought individually for as low as $1.99 (for a short story) or as high as $69.99 (for the Bible). An average downloadable audio book costs close to $20.

'Everybody has an audio book player in their pocket at this point,' says Anthony Goff, vice president of Hachette Audio, where sales have jumped by 31% this spring over last. 'It makes that much easier for the masses to try it.' Downloadable books made up some 60% of total audio unit sales in 2011, dwarfing CDs.

Audio book producers have been dramatically increasing their output. 13,255 titles came out in 2012, up from 4,602 in 2009, according to the Audio Publishers Association.

Audio books are no longer viewed as just an ancillary product to print. Some audio publishers are now attempting to rebrand narrated books as a distinct medium from print, labeling them as 'audio entertainment.' A handful of such publishers have started dabbling in original audio content, hoping to demonstrate that recorded narratives can hold their own as original works of art.

In the past five years, AudioGo has produced about 25 works exclusively for audio, including 'Baseball Forever,' a remix of radio broadcasts of some of the top moments in the history of baseball, 'Dreadtime Stories,' a collection of original horror stories, and a series of apocalyptic zombie stories. The company has also produced 10 full-cast productions that resemble old fashioned radio plays, a nearly extinct art form. They hired actor Val Kilmer to play Zorro in an audio dramatization titled 'The Mark of Zorro,' and recruited Stacy Keach, Mickey Spillane's Mike Hammer on TV, to voice the detective in an original Mike Hammer audio book.

Audible, the country's largest audio book producer and retailer, has been investing in original works as a way to showcase the dramatic potential of the form, and to provide exclusive content as HBO, Showtime and Netflix do in television. Audible has produced nearly 20 original audio books, mainly in fast-growing categories like mystery and science fiction. Audible, which Amazon bought in 2008, runs a subscription service for millions of members, who listen to an average of 18 books a year. Plans range from $14.95 a month, for one book a month, to $229.50 a year, for 24 books a year. The pipeline needs to keep flowing.

'If you run out of mysteries, people quit,' Mr. Katz said.