This paper describes a project dedicated to the preservation, promotion and fruition on the web of the rare books collection of Capodimonte Astronomical Observatory. The project, promoted by INAF-Capodimonte Astronomical Observatory, was supported by the Campania Region through European funds. The final act of the project was the publication of a bibliographical catalog, Le Cinquecentine dell’Osservatorio Astronomico di Capodimonte, addressed to specialized users (historians of science and bibliophiles).