When I’m indexing a book, that is the constant question.
The first time I ask, it’s the big question. I want to nail down the metatopic of the book early in the process. “Metatopic” in indexing means the briefest possible mile-up summary of what the entire book is about.
The metatopic is the structural center of the index, its magnetic north. Every index entry is implicitly filed beneath it. If the book is about ice climbing, there is no main heading for ice climbing, because the rest of the index would be its subentries. Instead you have main headings for techniques, tools, locations, and notable climbers.
In this case, the metatopic is Virginia Woolf’s thoughts on writing. Knowing that, I move on to ask what is this page, paragraph, sentence, word about? Why is it here? What else is it related to? Why might various readers want to look it up?