Complex sentence: Vasari's account of the "Mona Lisa" comes from his biography of Leonardo published in 1550,31 years after the artist's death, and which has long been the best known source of information on the provenance of the work and identity of the sitter.
Split into simpler sentences: Vasari's account of the "Mona Lisa" comes from his biography of Leonardo published in 1550,31 years after the artist's death. It has long been the best-known source of information on the provenance of the work and identity of the sitter.