Bracken, C.P. Roman Ring ([London: Cassell, 1968]) 169 p. Cassell Crime.
My copy lacks a title page so the publication information above comes from the Library of Congress online catalog.
A criminal organization in Italy steals rare books from libraries. One of them is a former librarian named Mario Signarelli. He steals books from a library in Rome called the Cantal. I can't find any evidence that such a place exists. Another theft takes place at St. Paul's College, where the book catalog is altered to reflect the worthless books that were substituted for the rare ones taken.