The best way to find the Dewey Decimal number of a book is to look it up by title and author in the Library of Congress online catalog. The Library of Congress, as the largest library in the world, has a comprehensive database of books in many languages. It is continually assigning Dewey Decimal numbers to books.
The Dewey Program at the Library of Congress has full time staff members devoted to assigning Dewey Decimal numbers to books in English, French, German, Spanish, Italian and Portuguese. It works with the Cataloging in Publication program to assign Dewey Decimal numbers to books before they are published. It also designates Dewey Decimal numbers to every book assigned a U.S. International Standard Serial Number, or ISSN.
The Dewey Decimal Classification system is a method of organizing the world's published books and other library materials. The unabridged print version fills four volumes, and the digital version is available on a subscription basis from a site called WebDewey. The system organizes all library materials, including fiction, according to field of study. There are 10 major classes. Each book is assigned three whole numbers which break it down into class, division and section. Decimal numbers create more intricate divisions. For example, a book on British history would be found in class 900, which is history, division 940, which is European history, and section 942, which is history of England and Wales.