"The Thirteenth Tribe", by Arthur Koestler (1976, published in the U.S. by Random House; published in Great Britain by Hutchison & Co. Ltd, London; published by Omni Publications, Palmdale, CA; Library of Congress; ISBN 0-394-40284-7):

The European Jews aren't even the religious or ethnic decendents of the Jews of ancient Israel! -- besides the 2,000 years of all the inevitable Jewish-&-non-Jewish intermixing in Europe and elsewhere!

"This book traces the history of the ancient Khazar Empire, a major but often forgotten power in Eastern Europe, which in the Dark Ages BECAME CONVERTED TO JUDAISM. Khazaria was finally wiped out by the forces of Genghis Khan, but evidence indicates that the Khazars ["the Jews"] themselves migrated to Poland and formed the cradle of Western Jewry."

"...This has led several historians to conjecture that a substantial part, and perhaps the majority of eastern Jews -- and hence of world Jewry -- might be of Khazar, and not of Semitic origin."