Huffington Post says:

Yesterday, the Independent Publishers Group (IPG) announced that Amazon has removed all their Kindle editions from the site, totaling around 5,000 books.

According to Publishers Lunch, IPG's president, Mark Suchomel, sent out an email alert yesterday stating, "I am disappointed to report that Amazon.com has failed to renew its agreement with IPG to sell Kindle titles."...Amazon turned off the buying button on the approximately 5,000 Kindle IPG titles because they refused to accept a revised set of terms regarding revenue from Amazon.

In an interview with paidcontent.org, Suchomel said, “We’re offering [the e-book sales terms] we offered last week, and somehow they think it’s not quite good enough."

IPG is basically a distributor made up of many small, independent publishers who otherwise would probably get lost in the battle for placement and promotion against the larger New York-based publishers. Amazon's new agreement—it involves steeper discounts in Amazon's favor and new co-op terms—kicked up a lot of dust when it was announced in December. (Co-op, if you're unfamiliar to retail terminology, is money that producers pay to retailers for better placement on their shelves or on their site. You don't see books on display at Barnes & Noble, for example, unless publishers pay for the advertising space.) We'll see how this fight goes; Amazon's not known for budging.