Heimskringla by Snorri Sturlason 1225 CE

"Heimskringla (Icelandic pronunciation: ​[ˈheimsˌkʰriŋla]) is the best known of the Old Norse kings' sagas. It was written in Old Norse in Iceland by the poet and historian Snorri Sturluson (1178/79–1241) c. 1230. The name Heimskringla was first used in the 17th century, derived from the first two words of one of the manuscripts (kringla heimsins, "the circle of the world").

Heimskringla is a collection of sagas about Swedish and Norwegian kings, beginning with the saga of the legendary Swedish dynasty of the Ynglings, followed by accounts of historical Norwegian rulers from Harald Fairhair of the 9th century up to the death of the pretender Eystein Meyla in 1177. The exact sources of his work are disputed, but included earlier kings' sagas, such as Morkinskinna, Fagrskinna and the 12th-century Norwegian synoptic histories and oral traditions, notably many skaldic poems. Snorri visited Norway and Sweden. For events of the mid-12th century, he explicitly names the now lost work Hryggjarstykki as his source. The composition of the sagas is Snorri's."

-summary taken from wikipedia

 

Source:

https://en.wikisource.org/wiki/Heimskringla