There are two stories (adult, or nearly) that I read in my childhood that I’ve been trying to track down for ages.
A short story of the fall of Ys. It ended with the common “Par-Ys” prophecy. I read it around 1957–8 in a book that was already physically old. I thought it was by Robert Louis Stevenson, but it apparently wasn’t.
A novel with two main characters, Irish, named “Sean” (I think) and “Marjory” or “Marjorie”. The remarkable thing about the book was that it was illustrated, but not with drawings; instead, it was all posed photographs, in sepia as I remember. I read it in 1958–9, once again in an already-old book.
If #1 was a short story, might it have been de Maupassant? I looked through the list if his story titles and nothing jumps out as being specifically about Ys, but I wonder if there is any connection between Ys and Mont St. Michel. Because he does have a story called The Legend of Mont St. Michel.
I found a book in my mom’s bookshelf. It was written in the 1920’s. The main character was a WWI Vet dealing with PTSD (shell shock). He’s living a aimless life and depressed. Memories of trench fighting haunt the character.
He often hikes in the Appalachian mountains and knows the hill people. The mountains helps calm him.
Things change with the arrival of a county nurse trying to get the children attending school. That puts her in conflict with the Hill people. Education is considered a waste of time and leads to hill people moving to the Flatlands to get jobs.
The main character reluctantly helps the nurse. She eventually gets kidnapped and he has to use his infantry training to save her.
I read the book at least a dozen times during visits home. Unfortunately I never wrote down the author and Title.
It was unusual for a post WWI novel. PTSD was not fully understood then. The conflict between the hill people and flatlanders was interesting.
I’ve heard that conflict still exists. Hill people (the authors term) endure a lot of poverty but are furiously protective of their culture and are suspicious of outsiders.
Here is a blog about photo-embedded literature with examples going back to 1892. There are hundreds of posts there. I looked a few of them but didn’t see anything like your book. If anyone would know, I’ll bet he would.