According to the Audio Publishers Association, audiobooks are now a billion dollar part of the publishing industry.

Taking results from two studies of the audiobook industry, the association noted that 3.8 million more audiobooks were sold in 2011 compared to the year before–generating a 2.6 percent rise in revenues. Here’s more from the report:

The total number of titles published in the audio format has doubled over the past seven years and publishers continue to increase their output to ensure that the most popular trade books are available in audio at the time the print book is released. Adding to the positive outlook is a 3.2% decrease in returns of audiobooks from 14% last year to just 10.8% this year. Returns as a percentage of overall gross sales have never been this low … The study shows that for audiobooks, the digital download format has eclipsed the CD format with slightly more than 60% of all audiobooks sold today being digital.