
Eaters of the Dead: The Manuscript of Ibn Fadlan Relating His Experiences with the Northmen in AD 922 (later republished as The 13th Warrior to correspond with the film adaptation of the novel) is a 1976 novel by Michael Crichton, the fourth novel under his own name and his 14th overall. The story is about a 10th-century Muslim Arab who travels with a group of Vikings to their settlement. Crichton explains in an appendix that the book was based on two sources. The first three chapters are a retelling of Ahmad ibn Fadlan's personal account of his actual journey north and his experiences with and observations of Varangians. The remainder is a retelling of Beowulf.
He blends a re-telling of the Beowulf story with the accounts of Ahmad ibn Fadlan [2] and his interactions with the Volga Vikings [3].
[1] https://en.wikipedia.org/wiki/Eaters_of_the_Dead
http://en.wikipedia.org/wiki/Ahmad_ibn_Fadlan
And I love the Eaters of the Dead. It's not high art, but I found it an enjoyable read (once you realise that most of the footnotes are fake).