Fiction? Only in the sense that the author purports to get into Lee's head from time to time and tell us what he's thinking. I believe he used Lee's writings as a basis for such passages, and maybe the recollections written later by others. Some of the discussions about the soldiers' reasons for fighting are probably fiction, but based on the sort of things soldiers actually wrote in their letters. But all the facts of the battle, and all the military characters, are real. The description of Pickett's Charge is absolutely breathtaking.