Every April, New York’s proud Scottish-Americans celebrate their heritage with the Tartan Day Parade, processing up Sixth Avenue in a sea of kilts, to the noble blare of the bagpipes. If you are thinking of attending the festivities next year, however, you might want to keep quiet about having read “The Invention of Scotland” (Yale University Press, 304 pages, $30), a punchy new book by the late historian Hugh Trevor-Roper. For as Trevor-Roper points out with ill-concealed glee, tartan and kilt, those universal badges of Scottishness, are about as authentic as Disneyland. Until the 18th century, no one north of the Tweed had ever seen a kilt; nor did the clans, as legend has it, distinguish themselves by the pattern of their tartans, until they were taught to do so by an enterprising clothing manufacturer. The Scottish costume is, Trevor-Roper shows, simply the latest example of an ancient national habit: the forging of tradition.