 Early in the summer of 1817 , Mary Shelley finished Frankenstein , which was published anonymously in January 1818 . Reviewers and readers assumed that Percy Shelley was the author , since the book was published with his preface and dedicated to his political hero William Godwin . At Marlow , Mary edited the joint journal of the group 's 1814 Continental journey , adding material written in Switzerland in 1816 , along with Percy 's poem " Mont Blanc " . The result was the History of a Six Weeks ' Tour , published in November 1817 . That autumn , Percy Shelley often lived away from home in London to evade creditors . The threat of a debtor 's prison , combined with their ill health and fears of losing custody of their children , contributed to the couple 's decision to leave England for Italy on 12 March 1818 , taking Claire Clairmont and Alba with them . They had no intention of returning . 
