@panzade,
Shaara based his novel on two books which were published in the 1950s in anticipation of the centennial. The first was
The Twentieth Maine, by John Pullen, published in 1957. (I read the Reader's Digest Condensed Books version in about 1958.) The other was
Pickett's Charge by George Stewart, published in 1959.
The Killer Angles and the motion picture both give a very skewed version of events, because
it was fiction.