I think I would at least include some sizable excerpts from SHiL... certainly the introduction and first chapter, and perhaps selections here and there. (If it's a large enough volume, then yes, the entire essay would be a good choice.)
As for his letters... I think there's so much to choose from dealing with politics, religion, philosophy, art, science, etc., etc., etc.; not to mention so much that is revealing of the multifaceted nature of the man, that it really is an overabundance of riches, when it comes to choosing. (Though I think I would include his November 16, 1916 letter to Rheinhart Kleiner, as it appears in Letters to Rheinhart Kleiner, as it is not only very informative biographically, with shows forth his sense of humor, his verve, and even some of his pricklier qualities -- I love his reconstructed conversation with Grandpa Phillips, for instance... I've known kids like that, and it's always a toss-up whether to be exasperated or absolutely tickled with 'em....)
His fiction and essays will be very difficult to choose from, as well, though I think "The Colour Out of Space" is an absolute must. (I'd also be very tempted to use The Case of Charles Dexter Ward, simply because of the fact it is a love-song to Providence in so many ways, and therefore highlights so many of his best qualities as a writer of weird and regional fiction.)
Still going to have to give this some thought, but one thing I'm fairly sure of: I'd also include The Poe-et's Nightmare as well. I'm one of those who actually feels that the two differing tones complement each other, and that even the cosmic section would lose by separation from its comic framework. (I read them separately, just to see how each came across, and found that the framing sections brought even more richness to it, partly by contrast, partly through other things, heightening the irony and the awe of the whole.)
