Seems that you are raising the issue of what is termed historiography; in the most general terms how can the reader know that history he is learning from a particular book approximates reality or "the truth". One answer is to read more than one author.

Specifically, in McPherson's volume on the American Civil War, in the first chapter, he goes into great detail as to how the Fugitive Slave Act of 1850 inflamed Northern public opinion against the south, for example imposing a fine of approximately ten thousand 2011 dollars upon anyone who obstruct the return of a fugitive slave. OTOH the military historian Keegan merely describes the 1850 Fugitive Slave Law in general terms, omitting such details.