In “NW,” Zadie Smith tells us a story about the stories we tell ourselves — stories about who we are and how we came to be in our particular places in life. Although the characters in her novel hail from the same few city blocks in northwest London, they represent a collage of varying histories and cultures, and their stories reflect this range of origin and identity. But even in this space of shifting boundaries, where divisions of race, gender and sexuality seem more fluid and malleable than ever, one boundary remains unassailable: that of social class. ...