While visiting Walmart’s website in search of an obscure book, I discovered that it was neither in stock, nor capable of being ordered. But I was referred—electronically of course—to an unexpected source: Barnesandnoble.com.

Taking heed of the ancient proverb the enemy of my enemy is my friend, Walmart has evidently struck up a curious yet most sensible pact with Barnes & Noble, which, like itself, runs an expansive chain of physical, rather than virtual stores.

An alliance between the two brick and mortar titans certainly makes sense for a variety of reasons: first, Walmart and Barnes & Noble.com share the fear of Amazon.com siphoning off an ever-greater portion of their sales. Nearly 45 percent of Amazon.com’s sales come from electronics and general merchandise, rather than books and CDs. This percentage has been creeping up in the last two years and will continue to rise as a result of organic growth and Amazon.com’s impending acquisition of Zappos.com, an online apparel retailer.

Second, both Walmart and Barnes & Noble are trying to jump-start their website sales, even as they manage increasingly complex chains of physical stores. Both Barnes & Noble and Walmart are experimenting with new techniques to drive web-browsers into their stores, and vice versa. For example, merchandise ordered online at Walmart.com can be picked up in or returned to one of its retail stores. This saves the consumer shipping costs and brings more traffic to the store. In an effort to reinvigorate its holiday traffic, Barnes & Noble recently entered the ebook reader fray with the Nook, an electronic bookreader that will debut this month. The Nook is replete with numerous features and gimmicks to lure book and CD buyers into Barnes & Noble stores.

Even as they build their online presence, Walmart and Barnes & Noble face challenges from which Amazon.com is exempt, for the former are forced to pay state sales tax for merchandise sold in locations in which they operate. Amazon.com, on the other hand, is exempt from sales tax in most states, on the basis that it does not operate physical stores in such states. This allows Amazon to retain an important price advantage over its rivals.

In my quest to find the obscure book that brought me to Walmart.com, I finally found it, brand new, in stock and ready to order, in a place available for all to see, facing out from its own virtual bookshelf—on Amazon.com.