1. "The Book" by Robert T. Graham: This book, printed in 1985, measures a minuscule 0.07 by 0.09 inches (1.78 by 2.29 millimeters), making it smaller than a grain of rice. It contains the entire text of the Lord's Prayer.
2. "The Old Testament" by the British Museum: This book, printed in 1964, is even smaller than Graham's book, measuring 0.03 inches (0.76 millimeters) in diameter. However, it only contains the words "The Old Testament" and is considered more of a "microscopic object" rather than a functional book.
3. "A Little Book of Miniature Books" by The Society of Miniature Book Collectors: This book, printed in 2013, is a collection of tiny books, including one that is just 0.04 inches (1.02 millimeters) tall.
Therefore, the smallest functional book is likely "The Book" by Robert T. Graham. However, the British Museum's "Old Testament" is smaller in terms of overall dimensions. It ultimately depends on what you consider the defining characteristics of a "book."