The book is brief, and tells a simple tale: a gentleman hires a magician to perform a task (after two earlier trials). There, that's it, that's the plot. Nowadays (not that now is better, but we're used to Now) that would be the set-up to the plot ... the book ends just as things are about to get interesting.
There is a sequel, the Day After Judgement, which picks up immediately afterward but which also somewhat disappoints.
Another fault--well, not a fault necessarily, but certainly a less-engaging choice--is that the horrors one might expect in a book about black magic are entirely played offstage, and only referred to. Imagine a Lord of the Rings with passages like "two weeks later they decided to go through Moria, where Gandalf died, unfortunately, fighting a Balrog. Still, with Lothlorien ahead, the Fellowship was somewhat optimistic." It's not a good thing.
There is a demon fashion-show/parade near the end which is worth a chuckle, but it's still not scary.
Blish' A Case of Conscience is much more compelling reading, so go there instead--unless you're a completist, or in the mood for a brief, non-unnerving look at the dark arts, circa 1967.
Note: a 3 star ranking from me is actually fairly good; I reserve 4 stars for tremendously good works, and 5 only for the rare few that are or ought to be classic; unfortunately most books published are 2 or less.