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....