There's a glaring anachronism at the center of most Amazon.com fulfillment centers: aisle after aisle of old-fashioned books. Amazon stocks these volumes for the many customers who still favor the tangible pleasures of reading on paper. Yet the company is relentless about increasing efficiency and has at the ready an easy way to remove some of those bookshelves: on-demand printing. Print on demand has been around for more than a decade. Publishers worry that a widespread shift to print on demand could, like the advent of e-books, disrupt their century-old business model. As the digital transition upends the industry, resistance to on-demand printing may fade. Smaller publishers that have already made the switch away from printing and storing their own books say it's well worth it.