I find that this is true about most non-fiction books I have read. Even when extensively cited, I often get the impression that the author is drawing large conclusions from small data.
Mortimer Adler says that history is closer to fiction than many other forms of books. It's not even data. Autobiographies in particular are rarely true, but not entirely false. It seems that the more data one has, the more likely they are to be biased in interpretation.
Got any recommendations? I am pretty deep into the Hamilton biography which seems very extensively researched and widely praised. It's an excellent story if nothing else.