Yes, the books in the Children's Literature Set are a required component for Foundations D, since they are an integral part of the lessons.
Children learn the all of the basic phonograms by the end of Foundations C, meaning that they now no longer need controlled texts because they have all the tools they need to decode virtually any word in their spoken vocabulary. Since they now have all the foundational tools, reading comprehension using real children's literature becomes a major focus of the curriculum once they reach level D. Reading fluency and comprehension are practiced throughout Foundations D with the books in the Children's Literature Set and with the nonfiction Foundations D Readers, as well as through various activities and games in the lessons. In every lesson, students read one of these books or a chapter in one of the chapter books.
Please note that you are not required to purchasing the set as a bundle from Logic of English, and many of the books may be easy to obtain locally or may already be in your collection. The books in the Children's Literature Set can be purchased from Logic of English individually or as a set, purchased from most book stores, or found at your local library. The literature set includes time-honored favorites like Little Bear and Are You My Mother as well as newer titles like Polar Opposites and Ling & Ting.
In the Foundations D lessons, many of the fiction books in this literature set are paired with a non-fiction text on the same topic to help students to explore genres and build background knowledge. The nonfiction texts are found in the set of eight Foundations D Readers.