Step 1: I am not a musicologist; I cannot write this book.
Step 2: I know more about music than I knew I did. It will be ok if I am very cautious.
Step 3: I can write this book in a better way than the hypothetical musicologist I had in my head as being better at writing this book, because I know how to write for people who don't know about music in technical terms. (And I know how to write.) A musicologist might be writing for other musicologists, but that is not what I want to do. I don't have to include fragments of scores in the book, because that would be intimidating to my readers, and a technical analysis of music, in the way that I would attempt it, would also be criticized by anyone with more technical knowledge than I have. So that would be a way of alienating all my readers at one fell swoop.
I can also see that musicologists borrowing from literary theory often don't know what they are doing. For example, they borrow from postmodernism and poststructuralism, but without realizing that that makes musical meanings more indeterminate. Thus they cannot really be as confident as they want to be about their conclusions.
I realize what I have been calling impostor syndrome in my own case is not really that at all. It is not that I think I am an impostor and everyone else knows what they are talking about. It is that we are all pretty much impostors. Some are the real deal, and I am still aspiring to that. Maybe I've hit that a few times in my career. But I see younger people in my field and think, no, you don't have impostor syndrome, you are an impostor. You are so far from being the real deal that you don't even know it yet.