The pleasure of reading The Sundial is in the quality of Jackson’s prose, the cleverness of the way she does character and plot through dialogue or limited narration, the way she can take almost any scene and infuse it with that little frission of dread and foreboding. I’m not as much a fan of it as I am of We Have Always Lived in the Castle, though there are commonalities; most of the characters are detestable, which is not something I get along with, and all but one or two are quite weak personalities, which means they don’t act much versus a single powerful character — which makes that character repellently appealing, but makes the rest of them seem pretty insipid.
Overall, it’s never clear whether this is meant to be horror, literary, fantasy/spec fic, whatever. It can be what you want it to be. What it is really is a story about people and the way they act and react, and how difficult it would be to find people who are really worthy of inheriting a new world. You don’t have to accept that the world is really ending, only that the characters believe so.
As you’d expect, there’s also a fine sense of place; the Hallorans’ home is a character in the story too. There’s a lot of description of it, which is all revealing of character and the history of the family, but if you don’t have the patience for it, that might seem quite slow.