The DBF (as they say) pointed me toward this fantastic blog, BibliOdyssey, which is a collection of archival images which are freely available on the web.
One of my favorite offerings is a group of scans from badass microscopist Christian Gottfried Ehrenberg, which you can see here.
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I've seen lovely hard-to-find books recently, collections of old botanical illustration, sea, bird, and extinct animal etchings that are just gorgeous. These are no exception!