There's a book about the theory of Khazar Jewish antecedents. "The Thirteenth Tribe" by Arthur Koestler.

1) Koestler wasn't a historian
2) The book had negiligible historical data to fall back on and is a classic case of the undergraduate student thesis having a pivotal couple of chapters padded out with commonly perceived history (what we would euphemistically call 'extensive backgrounding')
3) Koestler had a lot of hypothesis and conditionally framed sentences and paragraphs. He had very little that was solid in there.
4) Koestler explicitly stated that the Khazar theories should not be used as evidence pror or con Israel's right to exist as a UN recognized State.
5) There was some other stuff in there about racial perceptions which was not historical but very much Koestler.