NAPLES, Italy — For months now, the alarm has been resounding throughout the insular and competitive world of antiquarian books: beware of volumes bearing the stamp of the storied Girolamini Library in Naples. They could be hot.

The library’s former director, Marino Massimo De Caro, was arrested in May, accused of systematically despoiling the library he had been charged with keeping safe, stealing books and selling them on the open market or directly to collectors. And sharp sleuthing on the part of a professor at Georgia State University in Atlanta has raised questions about Mr. De Caro and the sale of other, possibly forged, books.

The unraveling of Mr. De Caro was precipitated by a chance visit to the library in March by Tomaso Montanari, a professor of art history at the University of Naples and a regular contributor to the daily newspaper Il Fatto Quotidiano. What he saw there — books piled haphazardly throughout and empty shelves, soda cans and garbage strewed about — immediately went into print, and that prompted a petition signed by hundreds of Italian intellectuals questioning Mr. De Caro’s appointment as director.

More important, Italian prosecutors took note and weeks later Mr. De Caro was in prison, accused of — but not yet formally charged with — embezzlement and conspiracy. Prosecutors say that in the 11 months during which he managed the library, Mr. De Caro stole hundreds of its volumes. Investigators found boxes of valuable books, many with the library’s stamp, in garages and private homes in several cities as well as in auction houses abroad. Four people said to have conspired with Mr. De Caro were also jailed.