Amazon has slashed the prices on a bunch of hardcover books in what appears to be a response to a recent promotion from Overstock.com to undersell the online retail giant on hundreds of thousands of titles.

Over the weekend, Amazon quietly boosted the discounts on many hardcovers in an attempt to match prices offered by Overstock during their promotional period. Some titles are being offered at 50% and even up to 65% off – much higher than the 40%-50% range that we’re used to seeing with Amazon.

“We’re having a great year and want to thank our loyal customers for making it happen,” said CEO Patrick Byrne. “For a limited time, we’ve priced our books at least 10-percent below Amazon.com’s prices, as of July 22, 2013. This move immediately affects hundreds of thousands of titles. As usual, if the final order is over $50, the whole thing will ship for free. Otherwise shipping is just $2.95 per order,” announced Overstock late last week.

The promotion was supposed to run for just one week, but Overstock says that it will run for longer. Just how much longer is unknown at this time.

You can see Amazon’s drastic price cuts in titles like Dan Brown’s Inferno and Gillian Flynn’s Gone Girl. Those novels have been cut 61% and 56%, respectively, in order to exactly match Overstock’s prices.

What Amazon has done is basically invalidate Overstock’s claims to be selling books at 10% lower than Amazon’s prices. Many titles are now offered at the exact same price on both sites.

The Shelf Awareness blog suggests that Amazon could also be feeling a bit emboldened by recent developments in the publishing world:

Some have speculated that Amazon is also emboldened to engage in dramatic price cutting–which hurts traditional bricks-and-mortar stores and feeds consumer perception that a fair book price is lower than its cost–by the Justice Department’s victory against five major publishers in the e-book agency model case as well as Wall Street’s acceptance of continued losses by Amazon for now in the expectation of retail domination–and major profits–eventually. This last point was seen most recently on Thursday, when Amazon’s quarterly results included a net loss and were below Wall Street expectations but did not provoke the usual rush to sell, as is the case with most companies whose results are disappointing.

Either way, Amazon has cuts the prices on many titles to historic lows for the company. And in some cases, this move has put the price point on hardcovers well below the e-book price.

[Shelf Awareness via GigaOm]