what is the lesson of the book

I don't think there is a particular moral other than to show the realities of the American Civil War. Certainly Crane tells us that there is no glamour in war,

"The men dropped here and there like bundles. The captain of the youth's company had been killed in an early part of the action. His body lay stretched out in the position of a tired man resting, but upon his face there was an astonished and sorrowful look, as if he thought some friend had done him an ill turn...He sat down and gazed ruefully. In his eyes there was mute, indefinite reproach. Farther up the line a man, standing behind a tree, had had his knee joint splintered by a ball. Immediately he had dropped his rifle and gripped the tree with both arms. And there he remained, clinging desperately and crying for assistance that he might withdraw his hold upon the tree." (Chapter 5)

There are themes like brotherhood and masculinity but lessons are simply interpreted by the reader.For myself I took from the story that war is hell and there are few heroes, only survivors.