Amazon to Buy Goodreads - NYTimes.com

Check this Wired article for more on the "Wool" phenomenon

"The deal is made more significant because Amazon already owned part or all of Goodreads’ competitors, Shelfari and LibraryThing. It bought Shelfari in 2008. It also owns a portion of LibraryThing as a result of buying companies that already owned a stake in the site. Both are much smaller and have grown much more slowly than Goodreads.
[...]
The deal did get some support from Hugh Howey, whose book “Wool” was originally self-published on Amazon and promoted through Goodreads and became a best seller. “The best place to discuss books is joining up with the best place to buy books — to-be-read piles everywhere must be groaning in anticipation,” he said in the companies’ news release."

Amazon to Buy Goodreads - NYTimes.com