AMERICANAH. By Chimamanda Ngozi Adichie. Knopf (United States) and Fourth Estate (Britain). 496 pages. $26.95.Ifemelu and her boyfriend, Obinze, are middle-class Nigerians, hardly "starving, or raped, or from burned villages" but still "mired in dissatisfaction." They consider Lagos a backwater and they want out.Chimamanda Ngozi Adichie's fascinating, infuriating novel "Americanah" recounts their respective experience of the United States and Britain and, later, of the Nigeria they return to.At 35, Adichie is already distinguished. Her 2006 novel "Half of a Yellow Sun," about the Biafran War, won the Orange Prize for Fiction, and in 2008 she received one of the MacArthur Foundation's "genius" grants. She writes beautifully polished, semiformal prose with a slight English accent.And she likes to argue. Ifemelu could be speaking for the author when, told that one of her opinions is "pretty strong," she shoots back, "I don't know how to have any other kind.""Americanah" jumps around in time and space, from Ifemelu's present at Princeton to her past in Lagos, to her early American years in Philadelphia and Baltimore, to Obinze's difficult period as a paperless foreigner in London, and finally back to Lagos, where, after many years, they meet again.