EL PAÍS put 119 popular books from 2025–26 through a detector. Pangram. Amazon.es, Casa del Libro, prizes. In 2019–20, across 56 titles, zero. Now: 35 with traces, 29 percent. Thirteen above 50 percent. Every book found to be “largely” AI-generated is self-published. Many are romances. That is the paper’s test. It is not a criminal verdict. It is not illegal.

The most visible case: Devuélveme mi país, Alvise Pérez. In February, No. 1 on Amazon. Pangram: 97 percent AI, 3 percent human. The tool does not say who pressed, nor whether the author gave notes or a whole novel. After midnight Pérez answered by email, then on social media: he wrote it “word for word”. The statement, the paper says, was drafted by his AI assistant. Two sentences. One No. 1.

Violeta Dark, a pseudonym: Mr. Sterling, 100 percent after Pangram. 109 books in five languages since 1 March 2025. Estrella Correa, 86. Pablo Poveda, 75. Leila Yoon, 53. No one answered the paper. Cercas, Muñoz Molina, Allende, Dueñas, Redondo, Planeta, Nadal: 0. Nine traditionally published books show traces, four of them nonfiction between 12 and 34 percent. Miguel Aguilar, Penguin: everything must start and end with a human.

Amazon requires disclosure of AI-generated content. It does not ban it. Even edited, it is still “AI-generated”. A spokesman: proactive and reactive measures, no details. An August paper, on 14,419 self-published ebooks: one in five above 25 percent AI. Among the Spanish self-published bestsellers in the EL PAÍS sample, 39. Pangram is not infallible. On 29 texts the paper generated, it caught them all. On 168 excerpts from 2019–20, no false positive in their test.

The paper recalls Open, Agassi’s memoir, written by J. R. Moehringer after 250 hours of conversation. One name is on the cover. Authorship, between a person and a machine, is an old debate. Now the machine is a model. Pangram sees patterns: “not X, but Z”, lists of three, a uniform rhythm. It does not see intent.

Readers already write on Amazon: “it seems written by AI”. On Monday the Caine Prize winner, about $13,000, was suspected of the same. The debate, the paper writes, will be closed by readers, if they choose. Until then, a percentage remains. After a percentage, the cover remains. Anyone who mixes them turns a detector into a judge.

Image: Raquel Riba Rossy, Librería La Carbonera, Barcelona, 30 November 2024, CC0, Wikimedia Commons. The photograph shows a bookshop, not the titles Pangram scored.

Sources consulted: How much AI is in Spain’s bestselling books? — EL PAÍS English.