The document discusses the Dewey Decimal System, which was invented by librarian Melvil Dewey to categorize nonfiction books into 10 main subject groups represented by 3-digit numbers. It explains that nonfiction call numbers are organized by subject number then author's last name initial, and provides examples of the 10 categories including 000s for general works, 100s for philosophy, 200s for religion, and 900s for history. The system allows books on the same subject to be shelved near each other alphabetically by call number.