Here’s the riddle for today: What 15,000 items on the shelves of the eight branches and bookmobile of the Springfield-Greene County Libraries can be read, but are not books?

And the answer is. . .audio books, a heavily used collection of 950 titles on cassette and 314 titles on CD, that are “a hot wave and here to stay,” according to Lynn Clark, the reference supervisor at the Library Center.

“Audio books are definitely one of our more popular items, and, yet, the industry has only just begun to transfer book titles to the audio format. There’s an endless number of audio books that can still be produced."

“People are hooked on the portability and the convenience of being able to ‘read’ a book while doing something else.”

There’s no riddle about the many things you can do while listening to an audio book. You can paint the house, wash the dishes, drive a truck across the country, work out at the gym. You get the idea.

And, of course, if you are visually impaired, audio books are more than just an alternate method of reading, they are a life-saver.

Audio books are as varied as their print versions, from Dan Brown’s blockbuster “The Da Vinci Code” and Arthur Agatston’s hit “The South Beach Diet” to mysteries such as Elizabeth Peters’s Amelia Peabody series to John Steinbeck’s classic “Of Mice and Men.”

You can check out in audio book format Patricia Cornwell’s “Cause of Death” read by Blair Brown, Lance Armstrong’s “Every Second Counts” read by Stephen Hoye or Patrick O’Brian’s "Master and Commander” ready by Patrick Tull.

Sometimes audio books can be the solution; for instance, you can keep the kids happy in the back seat with a children’s audio book.

Or, sometimes audio books can simply give you a sense of completion: you can get a book read during the 15 minutes you go back and forth to work every week. And, for avid readers, that’s certainly a reason to ‘read’ audio books.