One of the best classical bio books is THE LIVES OF THE GREAT COMPOSERS by the late New York Times music critic Harold C. Schonberg. The New York Times writes:
A smooth, closely woven sequence of brief biographies . . . set in a surrounding continuum of depth and breadth which reflects the author's solid musical culture, his erudition, his command of socio-historic background, and his long experience in every kind and degree of performance is exceptional.
Available here:
Harold Schonberg's Lives of the Great Composers
Harold Schonberg's Lives of the Great Composers