Nervous, painfully thin, introverted? Take heart. Such a child was conductor-composer Leonard Bernstein. And perhaps no other young American has so captivated the interest both of the musical and non-musical public. Here is a biography of that wonder-child, now the conductor of the New York Philharmonic: his Boston childhood, his Harvard days, his early struggles, and his phenomenal rise to fame with the City Center, on Broadway, and in the ballet. Here is ""Lenny"" in a biography which will be of particular interest to musical readers as well as to teen-age girls who have succumbed to his vivid charm.