I'm just sitting here realizing I need to re-read this whole collection with a notebook. I remembered the Dees thing and the Derry mention, but some of these other thematic links people are pointing out are making me see the book in a whole new light. Time for a deep dive!
I always come back to the atmosphere of place. 'Crouch End' creates a whole pocket of London that feels alien. 'Rainy Season' makes a specific Maine evening feel apocalyptic. This mastery of setting a mood through location became even more refined. Later stories like 'The Dune' (Florida coast) or 'Bad Little Kid' (a small town) are as much about the place as the plot. 'Nightmares' was the workshop where he perfected turning a location into a character.
There’s a meta-level in 'Umney’s Last Case' where the 'everyday setting' is literally the fictional world of a 1930s detective. For the detective character, his rainy, noir city is his everyday. The horror is the intrusion of his author, a man from the 'real' everyday world of the 1990s. So the story pits two different 'everydays' against each other: the fictional normal and the author’s normal.
King is playing with the layers of reality, showing that what’s mundane in one layer is fantastical in another.
The setting itself becomes a battleground between competing realities. It’s a brilliant, cerebral take on the idea, showing that 'everyday' is a point of view, not an objective truth. The horror is existential, about the nature of one’s own reality.