Somewhere there&rsquos an echo of R.L. Stevenson and his patient plodding Modestine. Brian Sewell, possibly reinvented as Mr B for the purposes of the book, dashingly rescues a donkey in distress out of Peshawar&rsquos traffic mayhem. The fact that he is part of a documentary film crew doesn&rsquot perturb him at all&mdashhe abandons the crew blithely on a moment&rsquos impulse, determined to