sentences:  He continued to work and re-work its components throughout his adult life, a period of more than 50 years.The "canonical" or mature form of Tolkien's narrative is often referred to as "Middle-earth" after his term for the inhabited part of the world in which most of his published stories were set.The mythological and cosmological background of Tolkien's published works was sketched in the posthumously published "The Silmarillion" (1977), and in Tolkien studies, a field that has been developed since the 1980s.