SEATTLE — Web retailer Amazon.com Inc. will buy online audio book seller Audible Inc. for $300 million in a bid to expand its reach in digital audio content.

Amazon said it agreed to buy Newark, N.J.-based Audible for $11.50 per share, a 23 percent premium over Wednesday’s closing price.

Audible’s catalog includes about 80,000 audio books, radio programs, spoken word selections and other digital content for download.

Amazon and Audible have been partners since 2000, when the companies, with Microsoft Corp., launched an e-book and audio-book store on Amazon.com.

Company executives would not comment Thursday on whether the partnership, which adds links to Audible on some Amazon book pages, has been financially successful.

In the last year, Amazon has bulked up its own digital content offerings, which include the Unbox movie download service, an MP3 music store and Kindle, the retailer’s e-book reader and accompanying store, which stocks 90,000 books, newspapers and other content and can play Audible files.

Amazon’s shares posted a nearly 5 percent gain Thursday in the wake of the acquisition news, even though analysts said the Audible deal itself isn’t likely to make much of a dent in Amazon’s profit margin woes in the near term.

“I don’t think it makes a big difference anytime soon, just based on Audible’s size,” Global Crown Capital analyst Martin Pyykkonen said in an interview. “This business of digital audio books has grown, certainly, but it hasn’t taken off.”

Audible hasn’t seen a profitable quarter for more than a year, despite its role as exclusive provider of audio book and other spoken-word content to Apple Inc.’s iTunes store, which accounted for nearly a third of its revenue in the third quarter of 2007.

For its part, Amazon has offered little insight into how its digital content businesses are faring. In its quarterly report this week, executives did not update investors on its Unbox or MP3 stores, and said only that they were having a hard time making enough of the $399 Kindles to meet demand — the device is currently out of stock on Amazon.com.

The company said its fourth-quarter profit more than doubled and forecast strong sales growth for the coming year, but a flat operating margin outlook dismayed analysts. Operating margin is an indicator of how much a company makes on each dollar of revenue.

“I have no issue that they’re not doing the right thing,” Pyykkonen said. “But in my view, at the end of the day, this is still a bloody competitive retail business.”