I was wondering the same..and got no answer. I _could_ ask an LLM but I prefer no to. On the other hand..this is the kind of question that is really hard to google..even if one got 2016 google. The word "book" and "font" in one query will yield stupid results that are not what i asked for.

So I went the way of lowest resistance, and to paraphrase, the LLM said, and it might be entirely wrong that Book is something between Light and Medium, a text weight that is supposed to be better readable for longer texts. So Something around 350/400 weight. Some fonts just use "Regular"

With my new "knowledge" I went about and googled and found this: https://gist.github.com/lukaszgrolik/5849599

So Strangers whos name is just a hex number, the LLM seems to be right. And that is your answer.