The Austrian National Library Google Books open-source image server
Published Nov 11, 2013 Updated Mar 19, 2014
The Austrian National Library scanned first 100.000 complete historical books from the beginning of 16th century until the second half of the 19th century within the project Google Books project participation. These unique historical books are now accessible online.
You can read the Press Release here:
The IIIF image API specifies a web service that returns an image in response to a standard http or https request.
About 600.000 copyright-free works are supposed to be digitized.