This is autofiction, a form of fictionalized autobiography. Autofiction combines two mutually inconsistent narrative forms, namely autobiography and fiction.
The protagonist of Marshlands is a writer who is writing a book called Marshlands about a reclusive character who lives in a tower and can fish out of his windows.
The narrator or protagonist is someone who is the opposite of reclusive, someone who has lots of acquaintances to whom he talks about Marshlands and other people behind the scenes are talking about Marshlands too and tell the narrator what they think about the book.
Even the reader can write part of the book by adding to the table of remarkable sentences on Page 88, making each reader’s version of the book unique. Isn’t that the idea, that each reader completes the book they’re reading almost always in a different way?