Marino Massimo De Caro, the director of the Girolamini Library, was among those in custody, as well as suspects from Argentina and Ukraine, police said.
In April De Caro took a leave of absence from the position after reportedly notifying police himself of the theft from the library, known for a vast collection of writings on theology and philosophy.
The library was subsequently impounded and temporarily placed under the auspices of the director of the Vittorio Emanuele III National Museum in Naples. Investigators have asked for international help in locating the 257 missing volumes, which may turn up on the black market.
First opened in 1586, the Girolamini Library contains roughly 160,000 volumes, 5,000 of which date back to the 16th century. Naples prosecutor Giovanni Melillo called the collection "seriously and perhaps irreparably dismembered and mutilated" after the theft.
Among various warrants in the investigation, a search was issued for property belonging to Maria Grazia Cerone, an assistant to Senator Marcello Dell'Utri, a former close aide of ex-premier Silvio Berlusconi.
The library, which was famously frequented by 18th-century political philosopher Giambattista Vico, adjoins the Girolamini church complex and convent. In addition to its trove of rare writings, the library is treasured for four well-preserved 18th-century rooms.