Well, Kazakhs and Khazars are not the same, even if there is some relation. But I am no turcologist (some colleagues always like to remind me), if I could recommend, e.g. during my studies Omeljan Pritsak was considered an authority on the topic of both early Rus' and Khazars. I also enjoyed this, even if there are many speculations, which may be discarded now. A great book on Avars was written by Walter Pohl, now it is also available in English.