LETTER FROM NEW YORK: The BBC Woman's Hour Broadcasts by Helene Hanff (HarperPerennial: $9; 140 pp.). Between 1978 and 1984, the author of "84, Charing Cross Road" presented a monthly commentary on life in New York for the BBC. Initially, she recounted the trials and pleasures of the Upper East Side of Manhattan: shopping, walking dogs, the beauties of Central Park in all four seasons, the rigors of living in a minuscule studio apartment. But Hanff's skill as a storyteller quickly took over, and her report evolved into a charming narrative about an eccentric group of friends that included a socialite fund-raiser, a resourceful doorman and a debonair English sheep dog. This merry memoir leaves the reader hoping that Hanff will return to doing radio commentaries--but for National Public Radio.