Twelve years after the publication of her award-winning first novel “White Teeth,” Zadie Smith returns to the racially mixed, multicultural northwest sector of London where she grew up in the 1970s and 1980s. Her newest novel is titled simply “NW.” It took more her than seven years to write, and she has likened it to a “problem play.” The plot revolves around four people in their 30s who come from the same subsidized housing project. Through their eyes, she explores the complexities of the class system in London and how people from similar backgrounds can come to have vastly different destinies.