> But that's just called research. If you want to learn about Sparta, it's pretty easy to find blog posts or reddit threads about what the best book is, or to find the multi-paragraph reviews on Amazon and Goodreads and see what they say, or professional book reviews of them.
> So you absolutely do have a way of knowing.
As long as you already know enough about Sparta to be able to tell whether those blog posts, Reddit threads, and Amazon and Goodreads reviews are legit or spam.
> What you usually don't have a reliable way of knowing is how good of a teacher on Sparta a local history teacher happens to be.
Are you saying you honestly can't think of a way to vet a local teacher? How about taking to their students?
> And the answer is usually, not good enough to write a book on it.
Which, coincidentally, can be said for most authors as well, but that doesn't stop them from writing a book and then flooding the internet with blog posts, Reddit threads, and Amazon and Goodreads reviews about how good it is.