The historical premise of this book, is the conversion of the Kingdom of Khazar, to Judaism in the 7th century. Rabbi Yehuda HaLevi [11th century] creates the fictional dialog between a Rabbi, and the King of Khazar, regarding Jewish Thought, Christianity and [General] Philosophy, which in turn lead the king and his people to convert. The book is highly polemic, and very insightful into the inner logic of Jewish Thought.

(This was a foot-note to the previous entry, but the comment was too long).