APA (7th ed.) Citation

THE WORLD BOOK.. (2007). The world book W-X-Y-Z volume 21. World Book.

Chicago Style (17th ed.) Citation

THE WORLD BOOK.. The World Book W-X-Y-Z Volume 21. Chicago: World Book, 2007.

MLA (9th ed.) Citation

THE WORLD BOOK.. The World Book W-X-Y-Z Volume 21. World Book, 2007.

Warning: These citations may not always be 100% accurate.