How do I tell where I am, and whether something I'm looking at is in one of the online books?

breadcrumb

At the top of each article, lesson, video or audio (or most other documents) you'll see a 'breadcrumb'. That's what web developers call them. It's a path to where you are. If you look at that path it should tell you where you are, where the article you're looking at is and where you'll find the article again.

It's a good idea to keep an eye on the bread crumbs as you look at documents on the site.

If something doesn't have a breadcrumber then it should be a page with many articles on it. However once you hone in on what you're looking for and you're reading the actually document, there should be some sort of breadcrumb at the top above the title.