Biblioteca Ayacucho names 63 books; ReadingRate holds 2 of them, listed here. The most approachable of them is Noli Me Tangere by José Rizal (difficulty 60/100, about 13 hours); the hardest is Quincas Borba by Machado de Assis (69/100).
Other lists: Penguin Classics Collection, Penguin Modern Classics, NYRB Classics, Everyman's Library, Library of America, Bibliothèque de la Pléiade, Heinemann African Writers Series, Oxford World's Classics, UNESCO Collection of Representative Works. All lists.