This note contains spoilers for the book.

Actually two stories set in the same universe, one previously published.

It's a really cool universe. It's described in horror terms, and I can see the point, but it doesn't use horror sensibilities. So I like it.

It's a secret history universe. WWII ended just before the really big occult weapons got into play, and secret agencies on the allied side are still keeping those secrets and working to prevent gateways and possessions. Magic turns out to derive from higher mathematics, and anybody who rediscovers the basic theorems gets drafted into the secret agencies.

And if they screw up, things eat an individual, a city, a planet, or the whole universe. In the second story they visit a universe where all energy has been eaten by a thing, and it comes that close to tricking them into letting it into our universe.

It's also got a wise-cracking first-person narrator. What's not to like?