I was recently reminded of this project ...
"The European Library webservice is a portal which offers access to the combined resources (books, magazines, journals.... - both digital and non-digital) of the 43 national libraries of Europe. It offers free searching and delivers digital objects - some free, some priced."
Please note that it is a beta/pilot version.
The Gabriel (GAteway and BRIdge to Europe's National Libraries) site used to provide access to these libraries. It has been integrated into the European Library site. The Gabriel site is no longer online.
"Advanced Search allows you to modify which collections to search, if you desire. The "Online books, images, maps, etc..." has amazing materials, but only from four libraries -- for now. (I retrieved images of WWI US posters, full-text scientific materials, and more. I found full-text from the 1600s to current materials.) There is also a section called "Treasures", which has scanned images of special collections from many national libraries.