Complex sentence: McGuffey Readers were a series of graded primers, including grade levels 1-6, widely used as textbooks in American schools from the mid- 19th century to the mid- 20th century, and are still used today in some private schools and in homeschooling.
Split into simpler sentences: McGuffey Readers were a series of graded primers, for grade levels 1-6. They were widely used as textbooks in American schools from the mid- 19th century to the mid- 20th century, and are still used today in some private schools and in homeschooling.