Some pages that might be of interest to you:
Gallica, bibliothèque numérique de la Bibliothèque nationale de France
Digital Books at the University of Chicago Library
Wolfenbüttel Digital Library - Herzog August Bibliothek
Karlsruhe Virtual Catalogue
Astronomical Books Online - Link list
Portal to the Heritage of Astronomy
http://www2.astronomicalheritage.net/index.php/regional-and-national