"Where can I find a complete selection of strictly objective US
Sorry, but I don't think that you can. "Strictly objective" is a term
that may apply to scientific reports but not to biographies, although
the individual biographers may feel that they are being "strictly
objective". The existance of biographies presenting a different view
of the individual demonstrates that objectivity is a figment of each
biographer's interpretation. The opinions of book critics may or may
not help recognize this.
Biographers choose to write their books because they believe that they
have additional new insight into the individual's motives or an
interpretation of the influences on the person.
"Scanning a few of the White House biographies, I also see little in
the way of critical information."
"Critical information" can be two things: critical = significant;
critical = criticism. Either way, the choice in including "critical
information" is of itself subjective.
This comment is no help to find what you are seeking, but I hope it
helps explain why a list of books that are "objective biographies"
would be a subjective selection.