Because C.S. Lewis deserves his own list, we're going to leave him off. And so does Chesterton, so I'm going to ignore him, too.
But we'll throw Sayers into the mix, if only because I'm not sure Tolkien, Charles Williams, and Owen Barfield wrote enough non-fiction to get us to five.
1) The Monsters and the Critics, by J.R.R. Tolkien. The 20th centuries most important piece of Beowulf scholarship, and Tolkien writing about fairy stories? Yes, and yes. It's a steep drop-off from here, but that's no insult to any of the other texts.
2) The Figure of Beatrice, by Charles Williams. It's not very widely read, except by Williams fans and those interested in Dante. But as a study of The Divine Comedy, it's both insightful and provocative.
3) A Severe Mercy, by Sheldon Vanauken. You'll weep when you hit "The Deadly Snows," which has some of the best prose I've yet read. It's just inevitable.
4) The Mind of the Maker, by Dorothy Sayers. See, I don't hate Sayers. Go here for a provocative study in creative illumination, but don't take your Trinitarian cues from her. Go here instead for those.
5) Creed or Chaos, by Dorothy Sayers. Sayers is at her best in short-essay form, and there are a few in here that pack a punch. If you can't get this version, then look for the updated one, Letters to a Diminished Church.
Yes, I've left off Owen Barfield and substituted 3(!) books by people who were not technically Inklings. Barfield simply has the unfortunate designation of being the most prominent member of the group whose work I've not yet read.
Feel free to play in the comments. How would you make the list?