I have read
The Hill of Dreams and
The Secret Glory, but it's Machen third and final novel,
The Green Round, that I love. It has connections with "N."
"N," I believe, refers to a London postal code area, not to mathematics.
[
en.wikipedia.org]
When you've read "N," you might be curious about a book I own but haven't read, don't know if it's good -- Chris McCabe's
Buried Garden: Lockdown with the Lost Poets of Abney Park Cemetery (2021), which has an element of a search for the park that might have inspired Machen. It also refers to the artists Samuel Palmer and Edward Calvert (and Wm Blake) so I had to have it.