The article discusses the recent book De Bibliothecariis by Mauro Guerrini, professor in Librarianship at University of Florence. Mauro Guerrini collected in this publication a selection of his papers written within 1987 and 2017, connected by the subject of a reflection about the role of the librarian and about how it is changing, as it emerges from the various papers concerning eminent Italian and foreign librarians who have made history of librarianship. The author of the article highlights also Guerrini’s great interest in cataloging and the organization of bibliographic information, relevant subjects in the scholar’s activity .