The Thirteenth Tribe

The Thirteenth Tribe (1976) is a book by Arthur Koestler, which advances the thesis that Ashkenazi Jews are not descended from the historical Israelites of antiquity, but from Khazars, a Turkic people originating in and populating an empire north of and between the Black Sea and Caspian Sea. Koestler´s hypothesis is that the Khazars converted to Judaism in the 8th century, and migrated westwards into current Eastern Europe (primarily Ukraine, Poland, Belarus, Lithuania, Hungary and Germany) in the 12th and 13th centuries when the Khazar Empire was collapsing.

Link to the pdf file (located through google search):

http://www.solargeneral.com/library/13-tribe.pdf