Most of the major publishing companies include mp3 audio books as part of their overall marketing strategies and audio books have been traditionally released on the same day or the same period as hardbound copies, which is usually before the paperback release. It can be quite interesting to note that while  mp3 audio books  are often cheaper to produce and distribute than hardbound copies or even paperbacks, they are often artificially priced higher than paperbacks as part of an overall distribution strategy. This is usually taken as a sign that the industry has yet to reach its full potential.