This seven-book series, which covers subjects from zoology to British poetry, becomes the most popular textbook in U.S. history with more than 120 million copies sold through the 1960s. The series uses repetition as a teaching technique and is the first textbook series to increase the difficulty levels as students progress through the books. Although the series is still used today in some school systems, popularity of McGuffey Readers fades after the 1960s.