Well there are a only a couple that actually stay that way, because sequels, but I mean they are at the very end like 'oh, things are better now because they couldn't get worse'
Beyonders by Brandon Mull, it ends with a good yay, everything is better epilogue.
Adventurers wanted, it's unfortunately not what the series was intended to be because the Author had a stroke before he finished and had to wrap up the next three books worth of plot lines in one book but it still gets the checks
Harry Potter, for as much as I bash this series it does have a decent ending
Lord of the Rings the ending hurts but it does give a things are better than they were before, but the world will never fully recover sort of feel that makes it bittersweet.
It does make me wonder is there a book that starts after a big world saving event and goes to show the full aftermath? I really want that now.