In ''What Am I Doing Here,'' Chatwin comes right out and tells us that five of the pieces are short stories, labeling each of them ''A Story'' lest there be any confusion. They're the weakest part of this collection, and they read more like outtakes from his other books than self-contained works of fiction. But many of the essays in ''What Am I Doing Here'' are examples of Chatwin at his best - part observer, part interviewer, part scholar. What brings them alive is his special talent for noticing life's strange, riveting details. He was a born Autolycus, a snapper-up of unconsidered trifles. What comes through in his last book is a life miscellaneous and on the move, traveled on foot, but never pedestrian.