I expect SS to be a genius in all things, and so I find myself demanding that his prose display a Henry Jamesian fullness of expression. But his sentences are sometimes a bit slack Unless the writer is Henry James himself, chekky, a Jamesian "fullness" of prose is usually an indication of pretension, not genius. (Your sentences are a good example of that.)
And "bombastic" in the subject line of this thread is, well, bombastic. (Or melodramatic, at any rate.)
I'm glad his prose is as it is. It's the way he talks, and the way he talks when he's excited and passionate about his topic.
And, sparrman, I'm not so sure I would blame "two different editors" for what you call the "repetition" on page 361. It's more like Sondheim wrote the book itself in two (or more) different passes: the first to include the lyric with occasional footnotes and the second to insert anecdotes. The footnotes are generally about lines he wrote that now irritate him. They are constant, throughout the book, as I imagine they are in his head: I imagine every single time he hears "I Feel Pretty": he cringes at what he calls the "sinful alliteration" of "fizzy and funny and fine." I don't find that footnote on page 361 is repetitious. I think the anecdote that follows amplifies it.
A third, fourth and fifth might have been to add the larger essays, the introductions and interstitial comments and, finally, the handwritten facsimiles and illustrations. It's like one of Suerat's large canvases that seems to be abstract but on close inspection has order and composition and beauty.
And I don't begrudge him his opinionated moments--in fact, I would have loved more of them. If all he had done was annotate his lyrics, it would have been wonderful...but cold.
The great achievement of the book is that it tells you the stories of his life the way a memoir would and sheds light into his creative process--both his enormous imagination and his intense self-criticism. But because it's not a memoir, he didn't have to "lie about himself a little," as Dmitri Weisman says in Follies.
I look forward to his second volume, which has such a perfect title: "Look, I Made a Hat."