Robert Frank’s “The Americans’’ certainly is a book, one that consists of 83 photographs taken during 1955 and 1956. It is also one of the defining documents of American culture, bearing witness to a larger, stranger, more mythic America than anyone had ever previously presented. The National Gallery of Art has organized a comprehensive traveling exhibition, “Looking In: Robert Frank’s ‘The Americans,'" now at the Metropolitan Museum of Art through Jan. 3.