This probably shows Nagato's complex about libraries. Life returned to normal, and the girl who loves books returned to the library. When reading, she will occasionally recall the experience of coming here with Kyon.
By the time they finished eating the casserole, their world had been restored, but Kyon's task had not been completed. Remember when he was stabbed to death by asakura? He didn't bring Nagato back to normal.
Finally, the man who picked up the gun and shot Nagato came from Kyon after eating the Christmas casserole, so Kyon had to go back to that night, watch himself get stabbed and come out to rescue him. Didn't the man say, "I'm sorry, I couldn't save you just now." Never mind, I was in pain, too. "