Amazon announced Thursday that it is acquiring Goodreads, the book-based social network founded by Otis Chandler in 2006. The purchase price was undisclosed, and the deal is expected to close in the second quarter of 2013.

Goodreads, which is based in San Francisco and launched in 2007, has 16 million members. The site lets readers keep track of the books they’ve read and want to read; rate and review books; and discover reading recommendations. Following the acquisition, Goodreads will remain headquartered in San Francisco.

Goodreads has served as a fairly “neutral” hub for readers until now — a place where publishers and authors can market and promote their books without being tied to a specific retailer. Until 2012, Goodreads sourced all of its book data from Amazon, but it then decided that the company’s API had become too restrictiveand switched its data provider to the book wholesaler Ingram. “Our goal is to be an open place for all readers to discover and buy books from all retailers, both online and offline,” Goodreads told me at the time of the switch. While being an “open place for all readers” may still be Goodreads’ goal, it’s now clearly tied to promoting books for sale on Amazon.

“I wanted to assure you that Goodreads and the team behind it are not going away. We have no plans to change the Goodreads experience and Goodreads will continue to be the wonderful community we all cherish,” Goodreads CEO Chandler wrote on the company’s blog. “Partnering with Amazon will help us focus on making Goodreads an even better place for readers.”

Goodreads is also likely to be less open with access to its data now that it has been acquired by Amazon. In the past, the company has shared information about how its readers discover and buy books and about their digital reading habits, presenting the data at conferences and in blog posts. This past February, for instance, CEO Chandler noted that ebook readers experiment with platforms — a significant percentage of Kindle users, for example, also buy ebooks from Apple’s iBookstore. All of this data is certainly useful for Amazon to have, but the retailer is not likely to see a reason for Goodreads to share the data with others (and with Amazon’s competitors).

Amazon acquired another reading site, Shelfari, in 2008.

This story was updated several times on Thursday afternoon.

Disclosure: Goodreads is backed by True Ventures, a venture capital firm that is an investor in the parent company of GigaOM/paidContent.