I have a plug-in in Umlaut which tries to look up available full text on Internet Archive.
It’s just an author/title search, so does result in false positives sometimes, but not too too often in my experience. Most of my Umlaut plugins use isbn/lccn/oclcnum lookup instead, but for a variety of reasons this isn’t practical for IA. In part because they’ve got a whole lot of full text without any such standard identifiers (project gutenberg for instance).
So anyway, this is in Umlaut, and then the Umlaut services are used in my OPAC to expose it. While it seems to work, it’s very rare that Umlaut finds something in IA that wasn’t also in Google Books/Hathi Trust.
But today I happened to come accross one such item, so I’ll put it here, for general interest, and so I myself can find it again when I need such an example!
The frog : its reproduction and development, Philadelphia: Blakiston 1951, [ Rugh, Roberts, 1903- ]
I have no idea how/why/if a book from 1951 is legally available in free full text. (which is why it’s not in GBS/HT). This one comes from the Biodiversity Heritage Library, perhaps they got a license from the copyright holder.
[PS: For whatever reason my brainless test search is often ‘frogs’, is how I came accross it].