The Observer quote on the front of the paperback edition of Stewart's latest book observes ''This is travel writing at its finest.'' Perhaps, but to call it travel writing is to totally under-sell it. This is erudition at its finest. Stewart has the background to do this: he had an international upbringing and followed his father in both the Army and the Foreign Office, and then (to his father's, bemusement, shall we say) became an MP. Oh, and he walked 6,000 miles across Afghanistan in 2002. A walk along the Scottish borders should be a doddle by comparison.