This book did for opera what Gombrich's work did for visual art: helped me understand the art form as a form, and the languages composers use to construct it. As with any art form, only a handful of examples will approach anything like the ideal: everything in its place, serving a purpose, contributing to the greater whole, resulting in something at once meaningful and also beautiful. And as the author's examples show, even the greatest composers have only one or two operas that approach such an ideal. This book put words to what I already felt as an opera-goer, and gave me some historical understanding of how the language (or perhaps more properly, tools) behind the form changed over time.
The book has gotten a lot of flak for being so harsh on Puccini. I think this is undeserved. First of all, it also criticizes Mozart, Verdi, and Wagner, to the extent the author deems it warranted. But second of all, I think we can all agree that pretty melodies are fine and worth listening to, but don't necessarily make a good four-hour theater experience. Turandot is just...kind of vanilla. And that's not even accounting for its problematic orientalism, which Kerman calls out too (and this in the 1950s!). It's like painting: an impressionist painting can be pretty. Does that mean it is the epitome of painting? Not necessarily. It needs structural balance, fine technique, an artistic viewpoint, and that know-it-when-you-see-it "thing" that makes a masterpiece.
For anyone interested on a well-argued and illuminating perspective on opera up to Stravinsky, read this. I only wish there were a sequel to this book, covering current opera composer such as Adams, Glass, and so forth.