Complex sentence: An important source for earlier conceptions concerning Mozart's method of composition was the work of the early 19th century publisher Friedrich Rochlitz, who propagated a number of anecdotes about Mozart which were long taken to be authentic, but with more recent research are now widely doubted.
Split into simpler sentences: An important source for earlier conceptions concerning Mozart's composition method was the work of the early 19th century publisher Friedrich Rochlitz. He propagated a number of anecdotes about Mozart that were long taken to be authentic, but recent research has made them widely doubted.