After dipping one toe into the DRM-free waters, two leading audio book publishers are set to ditch copy-protection technology altogether.

Following a successful trial with digital music store eMusic, Random House has told partners that it will begin offering all of its audiobooks as unprotected MP3s, reports the New York Times, unless retailers or authors specify otherwise.

In a memo [.pdf] sent out last month, Random House Audio told partners:

Beginning March 1st, we will no longer require that our retail partners use DRM when selling audiobooks via digital download. We believe that this move will allow for healthy competition among retailers targeting the iPod consumer, without posing any substantive increase in risk of piracy.

To reassure authors, Random House cites research conducted based on the company’s DRM-free experiment with eMusic in which watermarking was used to track any illegal distribution of audiobooks sold through eMusic’s store on filesharing networks.

The results: we have not yet found a single instance of the eMusic watermarked titles being distributed illegally. We did find many copies of audiobook files available for free, but they did not originate from the eMusic test, but rather from copied CDs or from files whose DRM was hacked.

Random House says its decision to go DRM-free will help to invigorate the marketplace for audiobooks since the company will be able to “sell not only through existing partners such as iTunes, Audible and Amazon, but we will also be able to foster new audio sales through any of our CD retailers who have web sites, and through emerging partners such as eMusic.” All of whom will be able to target iPod users.

The NYT also reports that the Penguin Group, the second-largest publisher in the U.S. behind Random House, will follow suit by making all of its audiobook titles available for download in the MP3 format on eMusic. Having originally declined taking part in eMusic’s trial, Penguin changed its mind after being inspired by the music industry’s partnership with AmazonMP3. “I’m looking at this as a test,” Penguin’s Dick Heffernan tells the Times. “But I do believe the audiobook market without D.R.M. is going to be the future.”