Cliffnotes, Sparknotes, and Bookrags are somewhat splotchy on this, but the main theme of the book is that there is no honor in war. Lee and Longstreet are polar extremes on the matter, the author inferring that men like Lee were responsible for the Confederacy losing the war and men like Longstreet winning; after all, Longstreet was personal friends with Ulysses Grant before the war, serving as best man at his wedding. Throughout the book, Lee hesitates to make the right decision largely because his honor prevents him from leaving the field.
Historically there is some truth to this. Lee had not always commanded the Army of Northern Virginia. Before him was a man named Joseph E. Johnston who, tellingly, Longstreet thought very highly of. Johnston felt that the only way the South could win was to avoid fighting as much as possible, making the North grow weary in the hopes of them giving up. If fighting had to take place, the South could take the defensive so as to minimize losses. Lee somewhat agreed to this strategy when he took command but he was not afraid to take chances. Case in point, Gettysburg was not the first time he tried to invade the North, the previous September he was narrowly defeated at the Battle of Antietam. Lee brought many victories, which made him popular, but doing so cost lives the South could not replace.
The necessity of Lee’s strategy is questioned in The Killer Angels and this motif of honor is repeated for the North. Notice that the Army of the Potomac was on its fifth commander and the troops within have lost confidence in their leaders. There is a prevailing feeling that “gentlemen” commanders are responsible for their losses. Buford comments that he fears that Gettysburg will be a repeat of what had happened so many times before: the Southerners will dig in and the Union leaders will attack regardless, simply because they are expected to do so. Chamberlain, like Longstreet, characterizes the face of modern war, releasing that victory is accomplished only through inhibition of honor. Historically, this came to the North shortly after Gettysburg with the arrival of Ulysses Grant. Like Longstreet, Grant was dark, broody, and lacking of social graces, but he had command whereas Longstreet could not overrule Lee.
Of course, there is more to your question than this, but the rest is history. The Killer Angels is an excellent primer to this period, but if you want complete answers, read up on the actual history. I answered only in relation to the book.