Jack Vance as a 13th century Armenian historian

 Or maybe it's the other way around.

Jack Vance was an eccentric science fiction and fantasy writer who had a keen sense of the vast variety of human experience and especially of religion and social arrangements.   I have no idea if he read the \

History of the Nation of Archers by  Grigor Aknerts’i but any reader of Vance might not care to bet against it.

Sophene Books
ISBN: 978-1-925937-52-7

Excerpt (translated by  Robert Bedrosian): 

As we heard from some of them, this [Mongol] people arose from their land of Turkestan and moved to some area to the east, dwelling there in extreme poverty for a long time as robbers and wild men. They had no religion except for felt images which they carried with them for witchcraft. They were in awe of the sun, as though it were a divine power. Then suddenly they came to their senses, very straitened by their wretched and poor life. They called upon the aid of God, creator of Heaven and earth, and swore a great oath to Him to be faithful to His commands.

By the command of God, an angel in the form of an eagle with golden feathers appeared to their chief named Ch’ankez, calling out to him in the dialect of their own language. [Ch’ankez] went and stood opposite the angel in the form of an eagle out in the open at a distance of an arrow shot. And then the eagle, speaking their own language, related all the commands of God.

It's available from Sophene Books - ISBN: 978-1-925937-52-7.  Thanks to medievalists.net for bringing this to my attention.