unglue (v. t.) 3. To make it legal for a digital book to be used, distributed, archived and preserved by libraries.

https://unglue.it/

From their email

"Six years ago, in response to a Knight Foundation challenge program, we put in a proposal entitled "10,000 Creative Commons Ebooks". We had compiled a list of 2,300 Creative Commons Licensed ebooks, and we thought that with some more resources we could identify as many as eight thousand more. We didn't get the grant, but we kept on adding free ebooks to the Unglue.it catalog using automated processes and by individual discernment. We added Public Domain ebooks from Project Gutenberg, over 60,000 of them. We added over 30,000 Creative Commons licensed books from the Directory of Open Access books. We added computer books and textbooks, about 5,000 total. By the end of 2021, we had 102,692 open access books in our catalog and had distributed 6.45 million ebooks."

Unglue.it Stats:
Unglue.it free ebooks by format:
pdf    102287
epub    73241
mobi    71757

Unglue.it free ebooks by license:
Public Domain    65102
CC0    33
CC BY    13239
CC BY-NC    1928
CC BY-SA    1771
CC BY-NC-SA    2344
CC BY-ND    676
CC BY-NC-ND    13928
GFDL    15
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The CC BY and CC BY-SA are free culture licenses and open for collaboration.