A lot of it's fiction.
So when I turned it up, I poked it with a ten-foot pole and I declared it primary source.
To be sure, like other primary sources, you need to read with an eye to What The Writer Was Thinking -- which, even if it was to inform people, was to inform them for some reason. Genre's important here. Epics, chivalric romances, and so on are all useful to getting into people's minds, but not for depictions of events. Even extraordinary ones.
Novels, OTOH -- novels can be useful for all sorts of details, because what the writer threw in for local color tends to be accurate.
Provided it's contemporary and local. Historical novels may give you a useful view of how people in that time viewed that historical era. The sorts of historical errors they make may give you some hints about their own practices, since it's so easy to assume that what you do is natural -- but only hints. Similarly with far-off countries -- The History of Rasselas, Prince of Abissinia is interesting in many ways but not as a guide to Ethiopia of the era -- and with characters purporting to come from far-off. (I've run across 18th century writers who satirize England by having a Chinese or what have you look upon it and find it inexplicable. Amazing how much they find inexplicable that a 18th century Englishman might find ridiculous.) And it helps if the writer is familiar with the precise environment; George Eliot (in "Silly Novels by Lady Novelists") about ridiculous pictures of high society.
There are advantages and disadvantages to it. On the disadvantage side is the sacrifices of strict naturalism to aesthetics. Judging how much a tale is melodramatic or even unusual is necessary. On the advantage side is that writers are trying to depict what is more or less typical in the setting; there's this thing about not being able to claim it really happened that helps. Sometimes.