Amazon, the dominant online bookseller, said late Thursday that it would buy Goodreads, the most visited social media site built around sharing books. The companies did not disclose a purchase price or other conditions of the sale, which will close in the next quarter.

With bookstores closing, Internet sites have become critical places for telling readers about books they might be interested in. This deal further consolidates Amazon’s power to determine which authors get exposure for their work.

Until the purchase, Goodreads was a rival to Amazon as a place for discovering books. Goodreads, which is based on networks of friends sharing reviews, was building a reputation as a reliably independent source of recommendations. It was also of great interest to publishers because members routinely shared their lists of books to be read.