I have a feeling Mushroom turns out to be the most reliableIt is and it isn't.
The basic timline of The Dance of Dragons was originally outlined in a chapter of Martin's book The World of Ice and Fire, and then expanded in Fire and Blood. However, both books are written as if they're authored by historians within the universe. They are not novels, they are semi-dry textbook accounts.
Thus the "what" is pretty well established as a basic plot outline. However, the author within universe relies upon three biased sources for his own history (a septon, a maester, and a jester), and has his own biases, so it's very much an "unreliable narrator" kinda situation.

The story also originally popped up as a series of shorts in the Rogues and Dangerous Women books, which I believe predate The World of Ice and Fire.