zlibrary is widely used because it offers a vast collection of ebooks for free, including academic texts and fiction. However, the legality depends on your country’s copyright laws—some books may be shared without proper rights. Compared to Project Gutenberg, which only hosts public domain books, ZLibrary provides a much broader selection, but not all are legally distributed. If you want to stay completely legal, Open Library might be a better option since it offers digital lending. Still, many people use ZLibrary for access to rare and expensive books.