Did I mention that one of the arts is learning to actually learn accurate information? I've read writers who read the primary source and made the serious mistake of believing them.
No one writes anything without a purpose, and that purpose is going to dictate what they write. Even those who write to illuminate their own time for future generations will illuminate it according to their own lights.
Not that you can't take anything at face value. Metaphors are usually safe -- which is to say, when I was zipping along through a work of St. Augustine's and ran across the casual statement that something was like a bethrothal, which lasted a year or two, I think it's safe to say that in his time betrothals generally lasted a year or two before the wedding proper.
But other than that. . . .
Where most writers go astray, I think, is in confusing prescriptive with descriptive writing. Whether an etiquette book or a law, people saying what people ought to do are not therefore descibing what they do do -- necessarily. Some people go overboard there, too, and say that rules are only laid down when a behavior is widely practiced, even if disapproved of. This I would think certain only for rules that are argued about and defended. A rule that is off-handedly mentioned may actually be the rule, too widely accepted to get into a frenzy about. OTOH, it can also be a dead letter.
And then, you often have to reconstruct the other side. Wide reading can only do so much -- especially in eras when reading everything you can doesn't really make it wide. But with, say Andreas Capullanus's Art of Love, you can read that love is impossible between a married couple -- and note that he argues for it at length and presents it as one of the cases settled by a Court of Love. Someone argued against it.