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.
Rare books admired by connoisseurs have been fetching increasingly higher prices. Last week, the Bay Psalm Book, the first book printed in English in North America, became the most expensive ever bought at auction when it sold for $14,165,000.
But questions of provenance have grown, thanks partly to the Naples case. Last year, the International Association of Antiquarian Booksellers issued a warning for buyers and sellers to check any Italian books from the 15th through 17th centuries purchased in the first half of 2012, in case they had been removed from the Girolamini.
Using techniques perfected in international organized-crime cases, Naples prosecutors are now focusing on rare-book dealers and collectors who may have bought works they probably knew had been taken from the library. They are slowly exposing the practices of the rare-book market, where deception sometimes reigns, prices can reach into the millions of dollars, and the trail often goes dead at the Swiss border.
“The international market absorbed, without batting an eye, books that they couldn’t not have known came from the Girolamini Library,” Giovanni Melillo, the Naples prosecutor who is leading the investigations, said in an interview this week. “The rule ‘don’t ask, don’t tell’ is what governs the rare-book market,” he added. Prosecutors have requested cooperation from across Europe, as well as from the United States and Argentina.
Mr. De Caro, 39, is a character who seems to have been conjured jointly by Jorge Luis Borges and the Italian crime novelist Andrea Camilleri: a rare-book lover; a figure in the nebulous orbit of former Prime Minister Silvio Berlusconi; a sometime consultant in the renewable-energy field; and, by his own admission to prosecutors in official court documents, the architect of the most successful forgery of a book by Galileo ever executed.
In March, in a previous trial with a smaller scope, Mr. De Caro was convicted of theft and conspiracy to sell some of the books. He is now serving a seven-year sentence under house arrest. Partway through the first trial, he began cooperating with prosecutors and admitted to taking some books from the library, but not all of those he is accused of removing.
Mr. De Caro said that he had wanted to sell them to raise money to restore the library and that its plunder had begun long before his tenure. Built in the 16th and 17th centuries, the Girolamini is a hybrid of a state library and a religious institution. It has now been placed under the receivership of the Italian Culture Ministry.
Prosecutors began investigating after Tomaso Montanari, an art historian, wrote an article in an Italian newspaper in March 2012 reporting disarray at the library. Mr. Montanari was called as a witness in a hearing on Monday, as was Filippo Maria Pontani, a classical philologist, who breathlessly recounted how he had visited the library to consult a manuscript in early 2012 and found the wood-paneled reading room in chaos.