One important concept in the development of number knowledge is the cardinality principle, or knowing that the last word counted refers to the total number of items within the set. One prominent theory suggests that children learn this concept by observing sets being both counted and labeled with the correct set size (e.g., 1-2-3! 3!), thus allowing them to form an analogy between the last word counted and the overall labeled set size. Unfortunately, children may not often hear this input during counting book reading. One factor that may influence the input children receive during shared book reading is the design of the book. In the current study I identified common features within counting books that may influence readers' focus on the cardinality of the set and the use of counting to see if it would affect children's exposure to sets being both counted and labeled with the correct set size, which Mix et al. (2012) call "comparison input." Similar to previous research I found relatively low levels of this comparison input. When considering the structure of the comparison input, more than half of children never experienced the ideal sequence of counting first then labeling the set size. This suggests that the low frequency and poor structure of comparison input may contribute to children's prolonged development of an understanding of the cardinality principle. Further, I found that the input that children experienced depended on specific features within the counting books. Children experience more comparison input with books that have a greater variety of numerical representations within the text and less comparison input when using books that make the number words salient (e.g., placing them at the start or end of the page). There was also evidence that books with more text elicited more comparison input. The current dissertation considered both children's and parent's input together across a variety of counting books, offering an in-depth analysis of children's experience during counting book reading. By identifying relevant features within books that lead to different types of numerical input it offers insight for parents and educators choosing counting books, and it provides the foundation for future research aimed at designing effective counting books.