In The Killer Angels, why will the battle be fought at Gettysburg?

Following the Army of Northern Virginia's decisive victory
at Chancellorsville in May 1863, General Robert E. Lee decided the time was right for a
second invasion of the North. Marching into Pennsylvania, Lee's original objective was
to reach the capital of Harrisburg or possibly Philadelphia; he then hoped to turn south
and invade Washington, D.C. from the north. Although Lee's army was at the apex of its
fighting strength, the untimely death of the irreplaceable "Stonewall" Jackson forced
Lee to reorganize his army, adding both Gen. A.P. Hill and Gen. Dick Ewell as new (and
inexperienced) corps commanders. Many of Lee's men were without shoes, so Gen. Henry
Heth (according to his memoirs), one of Hill's division commanders, sent a brigade into
Gettysburg to ransack several shoe factories believed to be located there. While there,
Heth's men noticed several brigades of Union cavalry, under Gen. John Buford, occupying
several ridges outside the town. Although Lee had ordered that no major action take
place without his instructions, Heth's men advanced upon Buford's men, thinking them
only a small force. The engagement became heated, however, and Lee eventually decided to
concentrate his entire army at Gettysburg. Unfortunately for Lee, his cavalry leader,
Gen. J.E.B. Stuart, "the eyes of the army," was absent on a raid, and the commanding
general was unaware that the Union Army of the Potomac was also advancing upon
Gettysburg.