Complex sentence: Early in the summer of 1817, Mary Shelley finished "Frankenstein", which was published anonymously in January 1818, and began working on "History of a Six Weeks' Tour", which was published in November 1817.
Split into simpler sentences: Early in the summer of 1817, Mary Shelley finished "Frankenstein", which was published anonymously in January 1818. She also began work on "History of a Six Weeks' Tour", which was published in November 1817.