"To great writers," Walter Benjamin once wrote, "finished works weigh lighter than those fragments on which they labor their entire lives." Conceived in Paris in 1927 and still in progress when Benjamin fled the Occupation in 1940, The Arcades Project (in German, Das Passagen-Werk) is a monumental ruin, meticulously constructed over the course of thirteen years--"the theater," as Benjamin called it, "of all my struggles and all my...