People are discovering the ability to listen to an audio book while they are doing other things, ie. multi-tasking.
“Between 2012 and 2015, sales of downloaded audio increased nearly 85%, according to the Association of American Publishers”
* the root of this issue is that CD audio books are very expensive and CD players never were very portable. This allows downloaded audio books to begin to encroach on the market. People love the illusion that they are multi-tasking, truthfully if you are doing something else odds are you are not concentrating on the audio book you are attempting to listen to. I digress, since people like to think that they are getting more than one thing done the audio books become a good way to absorb a book and accomplish a task at the same time. With cell-phones having more memory and being very portable they become a natural medium for this sort of absorption.*