American authors are up in arms about Amazon's new Kindle lending library, accusing the online retailer of "boldly breaching its contracts" with publishers as it exercises its "brute economic power".

Amazon.com launched its Kindle owners' lending library earlier this month, giving customers with an Amazon Prime membership (which costs $79 a year) the option to borrow one book a month for free, with no due dates.

With more than 5,000 titles available, including Michael Lewis's Liars' Poker, Suzanne Collins's bestselling young adult series The Hunger Games trilogy and Howard Jacobson's Booker-winning novel The Finkler Question, the books, said Amazon in its announcement, come from a range of publishers under a "variety" of terms. The "vast majority" are there following an agreement with the publishers to include the books for a fixed fee, while "in some cases", Amazon said it was purchasing the title under standard wholesale terms each time it is borrowed, "as a no-risk trial to demonstrate to publishers the incremental growth and revenue opportunity that this new service presents".

Literary agents were quick to condemn the project, releasing a statement saying "it is difficult to see how this programme is in the best interests of our clients".

Now authors themselves have also moved to criticise it, with US writers' body the Authors Guild describing it as a "mess", asking if any of the books in the programme are there legitimately and accusing Amazon of launching it to push the Kindle Fire as it fights an "unexpected ebook device battle" with Apple and Barnes & Noble.

The Authors Guild claims that the six largest US trade book publishers, Random House, Simon & Schuster, Penguin, HarperCollins, Hachette, and Macmillan, refused to participate in the lending library, with the next tier of publishers mainly also refusing. "No matter. Amazon simply disregarded these publishers' wishes, and enrolled many of their titles in the programme anyway. Some of these publishers learned of Amazon's unilateral decision as the first news stories about the programme appeared. How can Amazon get away with this? By giving its boilerplate contract with these publishers a tortured reading," said the Guild.

Amazon's belief that it does not need permission to include the books, that it is just required to pay publishers the wholesale price of the titles which are downloaded, is "nonsense", said the Authors Guild. "Publishers did not surrender this level of control to the retailer. Amazon's boilerplate terms specifically contemplate the sale of ebooks, not giveaways, subscriptions, or lending," it said. "Amazon, in other words, appears to be boldly breaching its contracts with these publishers. This is an exercise of brute economic power. Amazon knows it can largely dictate terms to non-Big Six publishers, and it badly wanted to launch this programme with some notable titles."

Publishers who have agreed to participate in the Kindle lending library, meanwhile, "do not have the right to do so without the prior approval of the books' authors", according to the writers' body, which is advising authors whose books are in the programme without their permission to contact their publisher to register their objections.