The author may have some impressive credentials, but the book itself contains at least one huge, inexplicable error.
Fisher points out, correctly, that Puccini's Tosca is based on a Sardou play. But this knowledge doesn't prevent him from claiming that "all the characters in Tosca were true personages in history." He goes on to give detailed biographical information about Tosca, Cavaradossi, Scarpia, and Angelotti as if they had actually existed, neglecting to mention that all of his material comes from Sardou's play, and that these are *fictional* characters.
It's an amazing gaffe. Anyone who relies on this book as an introduction to Tosca will end up sadly misinformed.