The Killer Angels by Michael Shaara (1974). The Pulitzer Prize-winning novel that follows the major commanders on both sides through the three days of the battle. Required reading before any visit — the auto tour will mean something different afterwards. Shaara's book inspired the 1993 film Gettysburg and rescued Joshua Chamberlain from historical obscurity.
Lincoln at Gettysburg: The Words That Remade America by Garry Wills. A short, brilliant analysis of the Address and the political and rhetorical traditions Lincoln drew on. Shows how 272 words rewrote the meaning of the war.
Gettysburg: The Last Invasion by Allen C. Guelzo (2013). The best single-volume history of the battle for general readers — confident, beautifully written, sharp on the controversies and on what was actually at stake.