For majority of people, especially children, book is an authoritative source. Virtually no one outside of research fields and very excited enthusiasts will check sources in depth to see whether what is shown in the book is true or not.

So, I think the point OP is making is that most people don't really check sources while learning things 'the conventional way'.

We have to distinguish learning from, let's call it, recreational education. If you're learning a topic, you're not going to be using a single book. Not even schoolchildren do that. Regardless of whether you consider a book an authoritative source, if you read multiple books those books will have to agree with each other, and where they don't you'll experience some confusion until you can resolve the contradiction.

If you're only ingesting information from a single source, be that a book, a teacher, or an LLM, then you haven't really learned, because your knowledge base is unmoored. You can't learn history by studying Tolkien's mythos.