It's hard to say. For one, the Citadel's library is mostly about the Regent and the general history of Achamoth, so it isn't the best resource for researching the Legacies. In the books that do talk about them, however, it can be hard to tell the difference between literal entities and metaphor. Most of the accounts that are strictly historical only refer to Aions that are facets of a Legacy, but the myths are a bit more diverse in how they present them conceptually. Sometimes the Legacies are referred to as if they are a single god that embodies the Legacy's traits perfectly, but that often seems like it's just a literary device to make a point. Some books of a more occult leaning make it sound literal, but who knows how accurate it is.

There isn't really any information dated to earlier than several hundred years ago.