It is important that your child is reading at his or her reading level. Reading a book that has been recited many times does not produce reading growth and can get boring for early readers. If a book is too easy and your child insists on reading it, he or she may lack the confidence needed to read a broader range of texts. Here's what you can say to encourage reading a more challenging book:
-"What do you like about this book that we could look for in a new book?"
-"You have done such a great job reading all the words correctly and understanding the story. Let's read another story so you can practice strategies if you get to a word you don't know."
-"Do you still enjoy the story? When I read, I read to enjoy the story or find out information. Let's find a new story that you will enjoy reading, and, maybe you'll learn something new!"
Reading a book that is too challenging is not comprehensible and creates frustration. We want to avoid saying a reader "can't read" a certain text. Instead, we can say:
-"This is a book you will enjoy more if we save it for later."
-"If we read this book and it seems too hard, we might miss some important parts."
-"Listen to how I read it. Let's save this book for later so you can read it like a good story."
Resource: https://www.readingrockets.org/article/selecting-books-your-child-finding-just-right-books
The five-finger rule is a way for your child to take ownership over deciding what books are a good fit and why.