I felt exactly this way a couple years ago, when all I had read was CoN, Mere Christianity, and Screwtape, but then I went through all of his theological/philosophical writings from then until now and all I have to say is WOW it was soooo worth it! So I would totally encourage you to do the same. He didn't write a "bad" book - they're all great.
and excellent quote from the Last Battle!
Here's one from one of his best books,
The Great Divorce. It comes at the pinnacle of the story:
"Here is Joy that cannot be shaken. Our light can swallow up your darkness, but your darkness cannot now infect our light...Can you really have thought that love and joy would always be at the mercy of frowns and sighs? Did you not know that they were stronger than their opposites?"
So powerful. And it reminds me almost exactly of the Last Battle, with the dwarves.