Krebs is actually encouraged by reading the history of his war. Hemingway writes Krebs
"sat there on the porch reading a book on the war. It was a history and he was reading about all the engagements he had been in. It was the most interesting reading he had ever done. He wished there were more maps. He looked forward with a good feeling to reading all the really good histories when they would come out with good detail maps. Now he was really learning about the war. He had been a good soldier. That made a difference."
Krebs feels better about himself after reading the history for several reasons. First, it's reality to him--if it's in a history book, then it must have really happened, and Krebs graps onto that. Ever since Krebs returned from the war, he lives a rather dreamlike existence. He has no real responsibilities; his parents treat him as if he never went to war and as if he's the same person as he was before he deployed, and he has told so many "lies" about his experience in Europe that his own reality has started to fade. The history book presents him with the undeniable truth of what happened, and this is a sort of grounding for Krebs. This is also why the maps are important to him. If he can see a record of the places he was, it makes it even more realistic that those places do indeed exist.
Secondly, the history book causes Krebs to feel that he has done something worthwhile. After still being treated like a teenager by his parents and not being able to relate to innocent American girls, Krebs starts to doubt that his service was worth all that he has lost (his innocence, peace of mind, meaningful friendships, etc.). No one in his hometown understands him, and even when he runs into other veterans he feels that he is the only one who was scared and who fought/killed out of a sense of self-preservation (see the beginning of the story). As he reads the history book, he realizes that "his" was is important enough for someone to write about and that he had a right to be scared.