The Delirium Brief
Fantasy, horror, humor
In which the Laundry goes pear-shaped.
Really, there's not much more to say about this one. It's black humor, heavy on the black. It features our original protagonist, Bob Howard, and shows the ways he has and hasn't changed. It's quite definitely on the arc towards the series climax.
If you like the series, you'll like the book. If you don't know the series, this is quite probably the worst place to start, since it's bringing together threads from not one but several previous entries. (The author suggests starting with The Atrocity Archives or The Rhesus Chart.) If you haven't tried the series . . . how does a paranoid cynical Lovecraftian bureaucratic comedy spy thriller satire pop-culture horror novel sound to you?