Open Library is an extensive catalog of published books. The goal of Open Library is to “list every book — whether in-print or out-of-print, available at a bookstore or a library, scanned or typed in as text.” The catalog contains over 20 million recorded books and the platform provides access to ~1.7 million scanned versions of books. If the book is not available, the Open Library entry is often linked to Worldcat or Amazon in order to access or purchase the book. It differs from Project Gutenberg in that, it is a catalog not necessarily an access platform.
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Image Source: Open Library