In Investigations, children's literature is recommended, not required. The majority of children's literature appears in the Family Letters (pdf) that focus on Related Activities to Do at Home. These letters include a list of books that support the mathematics of each unit, for parents (and indirectly for teachers and schools) to use to support their child's mathematical learning. In grades 3-5 these Family Letters are the only place where children's literature is suggested. In K-2, there are also a handful of units, most of them in Kindergarten, that suggest using a piece of children's literature to introduce an activity. In these instances, several literature suggestions are offered, and the book is considered an optional material rather than the centerpiece of a math lesson.