NAPLES — It was one of the most dramatic thefts ever to hit the rare-book world, the disappearance of thousands of volumes — including centuries-old editions of Aristotle, Descartes, Galileo and Machiavelli — from the Baroque-era Girolamini Library in Naples. Now, prosecutors at a trial here are trying to show how such a wholesale violation of Western cultural patrimony could have taken place.

The very man charged with protecting these treasures, Marino Massimo De Caro, a politically connected former director of the library, is accused of being at the center of a network of middlemen, book dealers and possibly crooked conservators — all part of what prosecutors say is a sometimes corrupt market for rare books in which much is spent and few questions are asked. Apart from Mr. De Caro, 13 others are charged, including a priest.

The full extent of the losses is not known — the Girolamini Library lacks a complete catalog — but prosecutors, with some bombast, have compared it to the destruction of Dresden during World War II. In 2012, the authorities recovered more than a thousand library volumes that were found in a self-storage unit in Verona traced to Mr. De Caro.

“This is the biggest books scandal to hit in the past 150 or 200 years,” said Fabrizio Govi, the president of the Italian Antiquarian Booksellers’ Association, adding that nothing of this scope had happened since the case of Count Guglielmo Libri, a 19th-century Italian collector who absconded with books on a grand scale.