We use Hathi a lot at Standard Ebooks as a source of scans to proof productions against. Archive.org has a somewhat better interface, but Hathi has a wider selection.
For the books that have been manually curated, multiple collections are indexed, including HathiTrust and the Internet Archive. Search will also fall back to showing hits from the "extended shelves" if a title is not in the catalog.
Try John Mark Ockerbloom's Online Books Page:
<https://onlinebooks.library.upenn.edu/>
For the books that have been manually curated, multiple collections are indexed, including HathiTrust and the Internet Archive. Search will also fall back to showing hits from the "extended shelves" if a title is not in the catalog.