One has to distinguish between the physical product audio book, that is delivered on cassettes and CD's and the digital audio book that can 'only' be downloaded from the Internet. But let's first look why the demand has increased that much:

The hectic lifestyle most of us are forced to lead leaves little time to sit down and read a book. Commuting, heavy work schedules, the kids, different daily chores use up a lot of time. There is simply not enough time left to pick up a book and sit down to enjoy it....

On the other hand we all have time in the day when we are not really busy mentally, where the brain is in 'idle' or has at least a lot of spare capacity. Whilst you are occupied with work that demands that you use your eyes (and hands), the mind would be glad to get decent and stimulating input. Just think traffic jam, weeding or cutting the grass, or any other manual chore and housework that use up hours every day, or jogging, working out - any type of sport that mostly occupies the muscles.

An audio book, read by a professional narrator who is telling you the story, is an ideal way to 'read' books that you wanted to read for a long time, but that lack of time did not permit. Story telling has a very long tradition and with the new technology, it is gaining popularity again.

"For many people, listening to an audio book is an opportunity to hear an author firsthand," says Mary Beth Roche, president of the Audio Publishers Association (APA) and president of Audio Renaissance, a New York-based audio book company. "It's like having the very best lecture series, not just in your own town, but in your own car or home." - and at your command, they'll read when YOU are ready to listen.

Audio books used to be thought of as limited special products for only the blind and sight impaired or for kids and lazy people who either can not or do not want to read themselves. This has dramatically changed over the last few years. A survey of the Audio Publisher's Association has found that the average listener to audio books is about 45 years of age and has an average yearly income of more than $50'000, at least a college education and often more advanced degrees. And these people are very Internet savvy.

In short: These affluent middle aged internauts are the ideal customers for on line audio book shops. Downloading an audio book is like picking up their email for these customers and they appreciate the ease of purchasing a new novel in form of a down loadable audio book, be it for their iPod, Zune or other MP3 player.

"Middle-class, well educated, and smart, that's a great demographic to have," says Eileen Hutton, vice president of editorial for Brilliance Audio, one of the oldest publishers of audio books.

Roche says there is no data on sales for the industry as a whole. "We are just beginning to send out surveys to compile sales information, but individually, members of the Association boast of annual sales increases in the double digits," Roche adds.

Audio book sales are increasing so fast that the audio book industry is not able to keep up with the demand. Creating an audio version of a book is more costly and time intensive than printing. More often than not, an actor or a professional narrator has to be hired to narrate the book, and there's the cost of the recording studio, a production crew, post production and packaging ( that applies to conventional audio books on CD - Down loadable audio books don't have that cost attached.). A large saving can be made by making the audio books available as down-loadable audio file. This is also reflected in the cheaper prices for audio books that are down loadable directly from an on-line audio book store.

The peak sales season for audio publishers are in the spring as Americans prepare for those long driving vacations or basking on the beach, and before the Christmas season, when people are shopping for holiday gifts. Sales start to be a lot more even though, as people routinely pick up audio books to feed their MP3 players.

Audio books are especially good for children who hate to read because of problems in school. That way they are familiarized with the adventure of using their own imagination to paint their mental pictures of what they hear, rather than just staring on the TV screen. Also, if they have the book and the audio, reading along can help to overcome reading problems:

"Librarians and teachers have been using audio books for years in the classroom, but parents aren't making the logical connection at home," says Amanda D'Acierno, director of audio publishing at Random House. "It's been proven that listening and reading go hand and hand. If children can hear the spoken word and follow along with the book that helps them. Listening is much more in tune with the way humans learn than sitting down and reading a book."

People who for some reason or other don't have the reading skills they should have, but are intellectually fit can listen to audio books to continue their education and listen to self help audiobooks, language course audios etc.