Tudor Parfitt, "The Thirteenth Gate, Travels Among the Lost Tribes of Israel" (Published by Adler and Adler, Bethesda, MD 1987)
"Twelve of the thirteen gates of Jerusalem correspond to the twelve tribes, through which the prayers of each of them ascend to the heavens... The thirteenth gate is for him that does not know which is his own tribe." Dov Ber the Maggid of Mezericz, Hasidic Leader died 1772. So begins Tudor Parfitt's "The Thirteenth Gate". Chapters include: "The Hostage Jews of Syria", "The Lost Tribes of India" "Singapore: A Dwindling Community," "The Secret Tribe of Zebulon" (Japan), "The Deliverance of the Tribe of Dan" (Ethiopia), and "Prisoners of Venda" (The Lemba of South Africa).
Some the communities dealt with are not "lost tribes" in the sense of shrouded origins and secret identies, but rather they are "lost" because they are fragile and not on the radar screen of world Jewry.
I found the chapter on Japan most interesting. Parfitt writes in a brisk, easy style. You feel that you are with him on his journeys. I highly recommend this book for arm chair travelers and "lost tribe" buffs.