"Lo prohibido (novela completa)" by Benito Pérez Galdós is a novel published in 1884-1885. Set in Madrid during the final years of Alfonso XII's reign, it follows José María Bueno de Guzmán, a wealthy bachelor who pursues forbidden relationships with his married cousins. His obsessive attraction to "the prohibited" leads him through adultery, social scandal, and financial speculation in a city consumed by luxury and ambition. This lesser-known work explores moral boundaries and family ties against the backdrop of Madrid's glittering yet corrupt high society.
Wikipedia page about this book: es.wikipedia.org/wiki/Lo_prohibido