This was the first book of Elizabeth Peters/Barbara Michaels's that I read but I still think it's one of her best. I love the way the characters view every development with careful, reasoned scepticism, even when being attacked by writhing smoke.
The manifestations are kept plausible, the build-up to the finale is great and the ending is nicely handled.
If you haven't tried any of this author's books under either name, please try this one. If you like it, please try any of the Amelia Peabody books, particularly the early ones. If you've met Amelia already, you could like this.
I love it. I must've read it at least 6 times and it still has me waking up wondering just what that noise in the hall could've been....