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Tangentially, there is a recent book--The Horse, the Wheel, and Language by David Anthony--that includes one of the most understandable explanations of how Indo-European linguistics works and how it might relate to archaeological research. Since Anthony is himself an archaeologist, there is a little too much information in the archaeological sections, but he makes a good argument for linking linguistic changes with archaeological changes, and his discussion helps to correct the impression most people seem to get, when one talks about the linguistic relationship between Tocharian and Celtic languages, that a bunch of geographically bemused Irishmen, no doubt loaded up on poitin, somehow wandered off into western China. (Briefly, what happened is that two bunches of people at roughly the same degree of distance from the original Indo-European language(s) and culture(s), wandered off in different directions. But Anthony explains it better and in more detail!)