Nearly 5,000 titles have been pulled from the Kindle Store today after book distributor Independent Publishers Group refused to change the terms of its contract with Amazon. In a letter to his company's clients, IPG president Mark Suchomel said:

Amazon.com is putting pressure on publishers and distributors to change their terms for electronic and print books to be more favorable toward Amazon. Our electronic book agreement recently came up for renewal, and Amazon took the opportunity to propose new terms for electronic and print purchases that would have substantially changed your revenue from the sale of both. It’s obvious that publishers can’t continue to agree to terms that increasingly reduce already narrow margins.

Suchomel did not reveal specifics of the proposed deal. However, late last year, Publishers Weekly reported that Amazon was negotiating with publishers for e-book deals that moved away from the agency model — 70 percent of the sale to publishers, 30 percent to Amazon — to a co-op model, which sees publishers and distributors paying upfront to have their titles in the Kindle store, as well as a pre-negotiated percentage of sales. Amazon's proposed co-op model would allegedly also apply to the sale of physical books, which it currently pays for up-front. We're not sure if the co-op model is directly related to IPG's titles being pulled, but judging by Suchomel's letter, it seems possible. IPG's titles are still available for physical purchase from Amazon.

This isn't the first time Amazon has removed a specific publisher's titles from the Kindle Store. In January 2010, Macmillan had its titles removed after it asked Amazon to increase the price of e-books to $15 from $9.99. Amazon eventually acquiesced and switched to a different agency model for e-books, but now it seems the company is looking for a bigger slice of its e-book sales.