"The Third Reader is used mostly for the information stage of reading. That is, the children should already be skillful at attacking new words and should be able to read quite fluently. Now, in this stage, they are using their reading abilities to learn new information and ideas. Many students can begin work in the Third Reader at about sixth grade. Studies in this Reader may spread over two or even three years.
"Work in this Reader is at the children's instructional level. That is, it is more difficult reading which children choose for recreation. It requires considerable effort to learn new vocabulary and think about difficult concepts. Much writing, speaking, and spelling study may be integrated with these reading lessons, but it is important, also, to teach language and grammar separately. Whereas in earlier grades the major part of language study could be integrated with reading, in higher grades children need the systematic approach that can be given in separate lessons.
"Upon completion of the Third Reader a child will have gained a large vocabulary. He will be familiar with numerous word roots and affixes by which he can decipher the meanings of new words. And he will be skilled in careful, detailed, studious reading."
Parent-Teacher Guide, page 62