Writers talk about writing a lot. Sometimes, their sharpest observations are made in places where you (and possibly they) think they’re talking about something else.
A quick trio of examples:
Ernest Hemingway, Death in the Afternoon. Ostensibly, it’s a book about bullfighting. But when I finally got around to reading it, I realized that Hemingway was also using bullfighting as a way to talk about all those issues of life and death and art that are too close and personal (not to mention too boring to the outside reader) to address directly.
Mark Twain, Life on the Missisippi. The chapter on training to be a riverboat pilot, and how it changed the way he looked at the river, applies in equal fullness to how training to be a writer changes the way you read books, and the way you look at life.
Robert Graves, The White Goddess. As anthropology/folklore/myth . . . frankly, it sucks. And it’s probably responsible for more vast tracts of woo-woo in modern genre fiction than just about any other work of alleged nonfiction. But as an exploration of how and why Robert Graves wrote poetry, it’s wonderful.