Shadow libraries (also pirate libraries or black open access) are online repositories of freely available digital media that are normally paywalled, access-controlled, or otherwise not readily accessible. Shadow libraries usually contain textual works like academic papers and ebooks, and may include other digital media like software, music, or films.
Anna's Archive, Library Genesis, Sci-Hub, UbuWeb and Z-Library are some of the most popular shadow libraries for books and academic literature.