Free for non-commercial use. I posted a batch of book images on Flickr, closeups, edge shots, stacked, pages, a couple with USB cords sticking out of the pages.
These four books are from my Aristotle collection, not all the oldest, but my faves in the group, including a 1550 Madius edition of Aristotle's Poetics, and the big volume below, Operum Aristotelis Tomus II Guillelmo Laemario edition published in Lyon in 1597--I think this is really cool: the front cover is stamped with the crest of the Duke of Saxony, and the back is stamped with the crest of the Holy Roman Emporer. Here's the colophon. The interior shot on the left is from the oldest book in my collection, a Latin trans. of Aristotle's Nicomachean Ethics, Politics, printed in Venice on June 25th 1516. More about this edition here.
Here's the thumbnail block. Click here to go to Flickr to check them out full-sized.