This comprehensive, copiously illustrated volume may contain everything you could possibly want to know about Scotland. The jacket copy describes the text as highly readable, and it is. The jacket goes on to claim, ''It is a tale of powerful rulers and of fierce clan chieftains. . . .'' This is stretching, melodramatizing what is, essentially a studious work, a careful tracing of the progress of Scottish history and the development of its arts and national character. Armchair travelers who long for the Highlands of ''Brigadoon,'' might be disappointed. For serious students of Scottish history, here is a rich lode to mine.