For 25 years Solomon Volkov’s purported memoir of Shostakovich has been debated by critics. Some are tired of the debate, and look for the book to convey greater truths. But this isn’t right, writes Alex Ross: “It isn’t enough for the memoirs of a major artist to have an ambience of authenticity. A book that subjected Picasso or Joyce to such manipulations would never have made it to publication. For some reason, though, music is treated as a childish realm in which fables serve as well as facts.”