Historia Regum Britanniae was written by Geoffrey of Monmouth around 1136. It was presented as a genuine historical account, and was treated as such until the middle of the 16th century. However, it is now treated as a work of fiction.
The story begins with the Trojans founding Britain more than one thousand years before the birth of Christ, covers Arthur’s rise to power, his marriage to Guinevere, and his eventual demise. It ends with the Saxon conquest in the 7th century.
Although the work is fictional, Geoffrey is credited as being the first to introduce non-Welsh speakers to the legend of King Arthur.
You can still find this book in print today.