Amazon is facing an unusual opponent in the battle for the lowest book prices online: Overstock.com, 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.