How Google Books & Digital Catalogs Index Literature
Google Books and global library union catalogs (such as WorldCat and the Library of Congress) index millions of volumes through cooperative digitization initiatives. Metadata ingestion pipelines extract machine-readable cataloging (MARC 21) records, Optical Character Recognition (OCR) text layers, high-resolution jacket scans, and international standard identifiers.
By organizing books according to Library of Congress Classification (LCC) and Dewey Decimal Classification (DDC), digital databases make centuries of literature instantly searchable by subject matter, chronological period, or exact ISBN identifier.