is the ending of the novel a happy one or sad?

I don't think that happy or sad works for this book. This was a tragic war.Millions of people parished including most of Liesel's family and friends. THe best we can say is that there is some consolation in Liesel surviving. She has kids and can relate her memories to another generation. There are lessons to be had in this so perhaps this means a little hope for the future.