Research libraries are facing an unexpected challenge: too many books. Despite digitization, bound collections continue to grow. Some libraries house their stacks offsite, which can create multi-day delays between request and retrieval. Last June, the Mansueto Library at the University of Chicago, which accumulates about 150,000 books every year, introduced a system of robotic stacks capable of holding 3.5 million volumes in one seventh the space required by conventional stacks. The trick: Librarians sort books by size and not by Dewey decimal system. Engineers from Dematic, a firm that builds automated parts and storage-retrieval systems for Boeing, Ford and IBM, designed a five-story underground storage area managed by five robotic cranes. Dematic has built 17 automated library systems worldwide, but the University of Chicago's is the most complex. The company has three more libraries under construction.