Book of Dede Korkut
The stories were first told orally before they were written as a book. There are many versions of the stories. Many dates have been used for the first written copies. Geoffery Lewis dates it fairly early in the 15th century. Cemal Kafadar said that it was no earlier than the 15th century.  Stanford Jay Shaw (1977) in his history of the Ottoman empire dates it in the 14th century.