One of the world’s most important libraries is housed in one of the city’s most impressive buildings, a Beaux-Arts masterpiece inside and out. Its collection contains some 49.5 million items, including more than 18 million books. None of it is allowed to leave the building, but materials can be examined by selecting an item from a computerized data base. The volume is retrieved from underground stacks and delivered in minutes. Library cardholders (anyone who lives, works, or is a student in New York State is eligible) can borrow items from eighty branches. Those lions lounging on the steps are by Edward C. Potter, who also fashioned the lionesses outside the Morgan Library.