The famous quote from Heinrich Heine actually comes from a play, a tragedy called Almansor which appeared in 1823, and he wasn't exactly talking about a contemporary event.

The story takes place after the conquest of Granada. At one point, the protagonist, Almansor, says that he's heard that Ximenes (Francisco Jiménez de Cisneros) has burnt books in Granada. Another character, Hassan, replies with the famous line: "Das war ein Vorspiel nur, dort wo man Bücher Verbrennt, verbrennt man auch am Ende Menschen", most likely alluding to the Inquisition.

The burning of books in Granada did actually happen, around 1499-1500. By order of Cisneros, between 4,000 and 5,000 books were burned in Granada (according to Juan de Vallejo and Álvar Gómez de Castro, his first biographers and the closest to those events), saving only the books about Medicine. They were later taken to the University of Alcalá, founded by Cisneros.

That the line is as useful to talk about the events in Granada in 1500 as it is to talk about Nazism in the 20th century is just a tragic yet revealing coincidence. There is no shortage of book-burning incidents or destroyed libraries throughout history.