The online retail giant said a "technical error" had caused some US customers to be "inadvertently sent copies".
Atwood's publisher, Penguin Random House, had instructed sellers to keep the anticipated sequel to The Handmaid's Tale under wraps until its official release on 10 September.
The Guardian reports that around 800 copies were shipped early, but only in the US.
The leak has angered independent booksellers who have, according to Publishers Weekly, contacted PRH representatives to complain about Amazon's error, and demand that the publisher sanctions the retailer for violating the strict embargo.
"It makes us look bad," said Rachel Cass, of Harvard Bookstore in Massachusetts. "This is bigger than just this book. Customers will see that people who ordered online got their books. They will come into our store and see that we don't have it yet. They won't know or care about embargoes; they will just see that Amazon can supply them a book and we can't. They might not come in next time."
An Amazon spokesperson said: "Due to a technical error a small number of customers were inadvertently sent copies of Margaret Atwood's The Testaments. We apologise for this error; we value our relationship with authors, agents, and publishers, and regret the difficulties this has caused them and our fellow booksellers."
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