Alex Ross could hardly have chosen a thornier musical era to focus on in his first book, but he’s more than up to the challenge of explicating the radically divergent styles (neoclassicism, atonality, minimalism) and personalities (Jean Sibelius, Arnold Schoenberg, Morton Feldman) of the past 100 years of “classical” music. Better yet, his narrative is ultimately one of hope and perseverance, not despair and decline as the more conservative critics tend to see it.