Curling up with a paperback may be a forgotten luxury for many thanks to today's busy lifestyles, but listening to a book on the go, while shopping or jogging, is fast becoming the new norm.

Gone are the cumbersome cassette sets that could cost three times as much as an old-fashioned book and often featured only excerpts to cut down on costs.

Now, audiobooks are just a click away and can be uploaded onto a smartphone for the same, if not lower, price as the print edition.

Mary Beth Roche, president and publisher of Macmillan Audio, says their reader feedback suggests many use audiobooks as a "multitasking tool," a way to "consume books when their eyes are busy."

For others -- whose work lives may involve long stretches at the screen -- they are a way to unplug.

"It's sort of nice to sit back, and relax and have a story told to you," said Roche.

Some 35,574 audiobook titles were released in the United States in 2015, according to the Audio Publishers Association, an eight-fold increase over five years.

That year, sales of books read out loud reached $1.77 billion, an annual jump of 20 percent.

According to the Author Earnings website, Amazon largely dominates the market -- even more so than for the print book industry -- with around 119,000 audiobooks sold per day in January 2016.

At the core of its earnings is industry leader Audible, which Amazon bought in 2008. The tech and retail giant also offers subscription deals, including one book per month for $14.95.

Most major publishers now have dedicated audiobook teams.

- A favorite in the car -

In the United States, audiobooks have long been serious business -- simply because Americans on average spend so much time in their cars.

"What we found out is that's a way for consumers to make that time in traffic, quality time," said Roche.

"Even way back when it was the cassette and then the CD, the number one place that people listen is in the car. We do find that a long car trip or a long commute is often what triggers someone to try an audiobook and to experience it for the first time and then they find other places where they can listen."

When audiobooks became available in digital format, people started using them during other activities -- when out shopping or jogging, performing household tasks or crafting.

- 'Lose myself' -

For thriller author John Hart, gyms and shops are not necessarily the best place to enjoy a book.

"But if it's a quite contemplative type of environment, driving or working in a quiet manner, it's probably a great way to experience these books," he said.

"Driving your car on a long trip I find for instance an audiobook is every bit as satisfying as sitting in a quiet room and reading and in fact it can even become more so completely immersive. I'd lose myself in the experience."

The audio rights for Hart's first book were sold to Recorded Books. After that, his publisher Macmillan sought to retain control of the audiobook as well as the print rights.

Anthony Goff, senior vice president at Hachette Book Group, noted that authors today have a much keener interest than before in the audiobook version of their works, sometimes suggesting readers or offering to do the reading themselves.

Most celebrities who recently published autobiographical works in the United States also provided recordings of the volumes, including Bruce Springsteen, Carrie Fisher and Bernie Sanders.

On average, audiobooks account for 10 percent of sales of the print version, said Roche. But the figure is greater for some genres, such as science fiction, fantasy and especially self-help works.

Roche estimates that at least 1,000 copies of a given audiobook must be sold in order to start making a profit. That's because they can be expensive to produce.

"We still count on print to lead the charge, but I do believe that audio sales are helping to expand the market and finding new fans for our authors," said Goff.