Pope John Paul marked his 84th birthday recently with the launch of a book sure to become a bestseller. In it he takes perhaps his last trip down memory lane, recalling carrying a sleeping bag in a grubby Polish train to his twilight years as pontiff. The book is a stream of consciousness from youth to old age, from vigor to fatigue, that covers mostly the years since 1958,when he was appointed Poland’s youngest bishop at age 38.