I'll begin by nominating The City and the Mountains by Eça de Queirós, from Portugal; the description intrigues me and I like that it mentions three countries of this region.
From Goodreads:
Born in Paris, Jacinto is the heir to a vast estate in Portugal which he has never visited. He mixes with the creme de la creme of Paris society, but is monumentally bored. And then he receives a letter from his estate manager saying that they plan to move the bones of his ancestors to the newly renovated chapel—would he like to be there? With great trepidation, Jacinto sets off with his best friend, the narrator, on the mammoth train journey through France and Spain to Portugal. What they discover in the simple country life will upend their own lives deliciously....
Goodreads -
Amazon Australia -
Amazon UK