I finished it a couple weeks ago and my feelings are still tangled. Honestly, the ending of 'The Last Battle' is a gut punch, but in a way that's supposed to be hopeful? Basically, Narnia ends. The world gets destroyed—like, literally unmade by Aslan—after the final showdown against the false Aslan and the Calormenes. All the main characters from the other books (except Susan, which is its whole thing) die in a train crash in England and are reborn in the 'real' Narnia, which is like Aslan's country.
It's meant to be this beautiful, transcendent thing where they realize their adventures were just the 'shadowlands' and now they go 'further up and further in' to the true, eternal realm. But man, the bit where Puzzle the donkey is forgiven gets me every time. The whole thing reads like a very direct theological metaphor, which some folks love and others find jarring after the earlier adventures.
It's a definitive closing of the book, no sequels possible. Leaves you feeling quiet and a bit sad, even with the promise of paradise.