I'm really a very weak musicologist. Wish it weren't so, but there's only so much you can dig deeply into in one lifetime, as if you hadn't noticed.
Look at the Coen brothers. All their minor characters are as interesting as their protagonists. If the smaller characters are well-written, the whole world of the film becomes enriched. It's not the size of the thing, but the detail.
Anyone can write one book: even politicians do it. Starting a second book reveals an intention to be a professional writer.