An eyewitness account within a diary is more authentic than a history book. It might also be more truthful. An opinion piece written in a paper back when the event happened is also more authentic than a history book. However, it might be less truthful.
Content is authentic when it is first-hand, contentious and unregulated. Content is truthful when it has a smaller audience, but content is biased by its audience even when it's only audience is its author.