The book is a fictional diary of an Arab emissary, Ahmad Ibn Fadlan, who travels north from Bagdad on a diplomatic mission and is recruited by a group of Northmen, by chance and quite against his will, as warrior #13 on a quest to rid their lands of a terror that treatens them.
On their travels he befriends the warriors and their leader Buliwyf, learns about their ways of life and traditions and helps them fight against the monsters, who turn out to be not what was expected.
This is the book on which the film "The 13th Warrior" is based on. I watched it before reading the book and was pleasantly surprised by the details and explanations missing from the film.
I've found the book very well written, the scared and confused character of Ibn Fadlan jumping from the page, and the footnotes are very amusing and informative.