Complex sentence: He edited critical editions of Haydn's music, principally the operas and masses, scarcely known at the time, and wrote his first book, "The Symphonies of Joseph Haydn", which appeared in 1955.
Split into simpler sentences: He edited critical editions of Haydn's music, principally the operas and masses, scarcely known at the time. His first book, "The Symphonies of Joseph Haydn", appeared in 1955.