Longstreet's invention of trench warfare is mentioned as new and innovative in the book, but trenches had been used since days of the Romans. In fact, Lee had used trenches himself and would do so again in future battles.
Longstreet was a strong proponent of the tactical defensive in warfare, a good idea in the right situation, such as at Fredericksburg. However, there is no one formula for success. Whereas Lee was constantly adjusting his strategy to accommodate new developments, Longstreet offered no such flexibility.
Also, it is a lot harder to be the person in charge and ultimately responsible. Longstreet, for all of his military talents, may have been better in a supporting role than in the lead one. For example, in a later campaign in Tennessee, Longstreet suffered one of the most complete repulses that the Army of Northern Virginia ever experienced in the war. He tried to shift the blame to others and commented to an aide that "he preferred being under General Lee, as it relieved him of responsibility and assured confidence." So the Longstreet of real life may not have been as perfect as the one here in the story.
Lee understood the merit of Longstreet's idea to swing around the Union Army to get between them and Washington. However, Lee decided on Gettysburg, not out of an uncontrollable emotional zeal, but because it was the safest and best choice given the circumstances he faced. Lee knew very little about the enemy's location and strength. He knew only that two of the seven Union corps were in front of him, leaving five unaccounted for at that point. Without Jeb Stuart to scope things out and lead him safely around the Union Army, Lee and his men risked stumbling into larger Union forces by accident. Since this is precisely what happened with Heth's forces on July 1, precipitating the whole battle, Lee was most likely eager not to repeat that mistake.
Again, there are countless writings on the subject analyzing it from all sides. In the end, Lee made the decision he did based on what he thought was best at the moment. Second-guessing by subordinates who didn't bear the responsibility for the decision, or Monday-morning quarterbacking, is always easier.
Certainly, Lee may have been wrong, and Longstreet right. General Eisenhower, when questioned about his opinion of Lee's tactics at Gettysburg, commented, "Why he didn't go around there [Little Round Top], I'll never know." And Eisenhower, responding to General Montgomery, who commented that he'd never have fought the battle the way Lee did, said, "If you had, I'd have sacked you." So there are certainly modern military experts who had doubts about Lee's choices, adding fuel to the idea suggested by the novel that Lee was obsessed with attacking at all costs.
However, it has also been noted that General George Patton's bold thrusts in World War II with his Third Army were the direct result of studying Lee's methods. Was Lee mad to stay and fight, or was he correct given the facts he had at the moment? The bottom line is that it is a very subjective question, and the portrayal in this novel is weighted heavily in Longstreet's favor. The reader needs to be aware that this may or may not be right and not to accept the novel's portrayal as the final word on the subject.