Old Book Art was started in October of 2007 with the intent of sharing out of copyright images from books that passed through John's bookstore, ZephyrusBooks. The number of books he has cataloged hasn't quite reached a hundred, though you will find numerous individual prints from these books (more than 4.600 as of October 2011).
The material in this online collection includes illustrations John scanned himself, but mostly material that other people have shared with him, and it also includes material from sources with online archives such as the Library of Congress. While some of the illustrations may be available elsewhere, there is often a fee involved, or the resolution quality is low, or they are archived in a format that is hard to access for many people. Here you can look at them and even use them without any cost (though donations are more than welcome).
Want a taste? Just hop on over and have a look yourself and if a look alone isn't good enough for you, you may also opt to head over to the shop and maybe buy a print or two.