"Unbiased" is a tricky word to use here, because typically it just means a high-quality, reliable source. But what I think you're looking for is a source that is high quality but intentionally resists drawing conclusions even when someone trying to be accurate would do that - it leaves you, the reader, to do the conclusion-drawing as much as possible (perhaps at the cost of reliability, like a sorcerer who speaks only in riddles). Certain history books are the only sources I've thought of that really do this.