Erich Auerbach wrote his classic work Mimesis, a history of narrative from Homer to Proust, based largely on his memory of past reading. Having left his physical library behind when he fled to Istanbul to escape the Nazis, he was forced to rely on the invisible library of his mind. Each of us has such a library—if not as extensive as Auerbach’s—even if we are unaware of it. In this erudite and provocative book, William Marx explores our invisible libraries—how we build them and how we should expand them.