Shiho Fukada for The New York TimesThe recently discovered manuscript for Beethoven's "Grosse Fuge." Heather Carbo, a matter-of-fact librarian at an evangelical seminary outside Philadelphia, was cleaning out an archival cabinet one hot afternoon in July. It was a dirty and routine job. But there, on the bottom shelf, she stumbled across what may be one of the most important musicological finds in years. It was a working manuscript score for a piano version of Beethoven's "Grosse Fuge," a monument of classical music. And it was in the composer's own hand, according to Sotheby's auction house. The 80-page manuscript...