Podcast content has moved over to the pay-per-download world following Apple's announcement that it will charge $9.99 to download the 20-episode audio book of Scott Sigler's EarthCore from iTunes.

The world's first "podcast only" novel, EarthCore was published in March 2005 as a free weekly serialised audio book podcast.

Before it was completed six months later, the podcast generated 10,000 subscribers and was published as a print novel by Dragon Moon.

The project came to the attention of Mike Warden, owner of indie music label Conquer The World Records, who described EarthCore as "really cutting-edge stuff".

"With the development of iTunes as a distribution model, we saw an opportunity to expand into the audio book market," he said.

"EarthCore is the first audio book put out by an indie label, and the first ever podcast commercially made available for sale anywhere, let alone on iTunes. "

Warden discovered EarthCore because the audio book features music by Detroit band The Transfer which is signed to the Conquer The World label.

"I started listening to the book and I couldn't put my ears down, so to speak," said Warden.

"I enjoyed the science and the suspense of the story, but Scott puts so much passion in the way he vocalized the characters of his own book. He's like a punk rock author; he just did everything himself."

Scott Sigler added: "Now that it's finished, there's over 15 hours of unabridged audio book sitting there. And I stress the word 'unabridged'; it's exactly the same book you can buy on Amazon. The exact words of the author, read by the author."