Complex sentence: The Köchel catalogue is the extensive catalogue of the works of Wolfgang Amadeus Mozart, and Einstein had such a depth of familiarity with Mozart that he had something pertinent to say about every piece Mozart wrote.
Split into simpler sentences: The Köchel catalogue is the extensive catalogue of the works of Wolfgang Amadeus Mozart. Einstein was known to have had such a depth of familiarity with Mozart that he had something pertinent to say about every piece Mozart wrote.