The building also houses the historic library of the Observatory. In the library scientific journals and very important books are found written by Apollonius Pergaiou, Chrysanthos Notaras etc, since the 18th century.
You can search the historic library online catalog by visiting the page: http://www.noa.gr/museum/library/opac/
Some book pages:
Pages from “Introduction in the Geographical and Global” by Chrysanthos Notaras, 1716, Paris