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Amazon (s AMZN) is facing an unusual opponent in the battle for the lowest book prices online: Overstock.com (s OSTK), which last week announced a promotional campaign to sell its books “at least 10 percent below Amazon.com’s prices.” While that promotion was only supposed to last for a week, Overstock has decided to continue it for longer.

In response, Amazon is slashing its own prices on print books. In a special report over the weekend, trade publication Shelf Awareness noted that Amazon has begun “discounting many best-selling hardcover titles between 50 percent and 65 percent, levels we’ve never seen in the history of Amazon or in the bricks-and-mortar price wars of the past.” Publishers Marketplace, however, notes that Amazon has offered similar discounts in the past when matching promotions from retailers like Walmart (s WMT).

In some cases, Amazon’s hardcover prices are below the prices it’s charging for Kindle books. Overstock does not sell ebooks.

Here’s a chart with some sample prices, current as of Monday morning — and evidence that Overstock hasn’t been able to maintain its promise to sell books for 10 percent below Amazon’s prices, as Amazon is generally matching them:

Book Author Hardcover list price Amazon price Overstock price Kindle price Inferno Dan Brown $29.95 $11.65 $11.65 $12.99 Sisterland Curtis Sittenfeld $27 $12.76 $12.76 $12.99 And the Mountains Echoed Khaled Hosseini $28.95 $12.04 $12.04 $10.99 Second Honeymoon James Patterson $28 $12.59 $12.31 $9.99

As far as I can tell, Amazon is only offering the particularly deep discounts on books that Overstock also sells. The Cuckoo’s Calling — written by J.K. Rowling under the pseudonym Robert Galbraith — isn’t available at Overstock (retailers have been scrambling to get print copies of the book). At Amazon, the hardcover is selling for $15.20, a fairly customary (for Amazon) discount of 42 percent.

Amazon declined to comment. Overstock spokesman Judd Bagley told me, “It’s not yet known how long the promotion will last. It won’t be forever, that much we know. But it isn’t limited to just a week, either. We’re just watching to see how our customers respond and so far they’re responding very favorably.”

Yet Overstock’s Top 100 list doesn’t reflect a favorable customer response, if there has been one. On that list, I didn’t see a single new bestseller — rather, most of the books on the list are old or niche titles like It’s Called Dyslexia and Word Smart for the GRE. Overstock also isn’t promoting the deeply discounted titles on its books home page; if you want that discounted copy of Dan Brown’s Inferno, you have to search for it.

Overstock is largely known for offering discounts on home goods and furniture, and shoppers simply may not think to go there for books. Amazon’s shoppers, however, are certainly benefiting from Overstock’s campaign.