The History of the Jewish Khazars provides a very interesting look at the Arab-Khazar Wars, as well as an attempt to explain the rise, conversion, and fall of the Khazar Khaganate. Dunlop's attempt to shed some light on this obscure civilization falls flat, although through no fault of his own. Despite assessing sources written in Greek, Arabic, Hebrew, Syriac, Armenian, Georgian, Russian, Persian, Turkish, and even Chinese, he still openly admits the unfortunate truth: we just don't definitively know a whole hell of a lot about the Khazars, and a myriad of sources helps very little when most of them disagree with one another.
This book is definitely worth reading if you want to know everything historians have uncovered about the Khazar Khaganate, it's just a shame that the truth about much of the origins and demise of Khazaria remain unclear, even to experts.