During a campaign, reporters can and do see many of the juicy moments. Not all, but a lot. Campaigns are big and sprawling and chaotic. They can't afford to freeze anybody out, notably starting with the press. Everybody wants to talk about what's going on, and many people have seen some interesting version of reality. So campaign books can have a relatively high "saw it myself" quotient, and in general they are believable.
Once an Administration begins, however, the available vignettes are more like the five-remove puppet show.
They represent administered rather than observed truth. By definition, reporters aren't there to see the big moments; also by definition, the people who are there to see have distinct self-interest in distributing certain versions of reality. By the time historians get around to sorting the evidence, they have a better chance of weighing the biases of the various early accounts. But when you read "inside" tales of Administrations still in power, bear in mind the back-story, motivation, and stylized kabuki-esque rituals behind any anecdote therein.