I confess that I've looked at the answers to this question for a lot of European countries by now, preparing a number of these posts. There aren't many where the top book is as far ahead of the field - on both LibraryThing and Goodreads - as it is in this case. And there are even fewer cases where it's a book I simply hadn't heard of. But this is one of them. The most widely owned book by a long way (among both LibraryThing and Goodreads users) which is set in Wales is:
I am of course familiar with the first of these. The second is the opening novel of a historical fiction trilogy, which has clearly done well while passing me by.
Fourth place in LibraryThing tags, and second on Goodreads tags, is the one I would have immediately thought of: