Through all my reading and video watching over the summer this is what I have learned about book boxes and filling them.
1) each student has 5 books at a time
2) each student needs 3-4 "just right" books and no more than 2-3 look books (this is specifically for kindergarten, more advanced readers will not have any "look books" in their boxes)
3) have students read their "just right" books before they read their "look" books (place a post-it on the back of the "just right" books and have your students make a tally mark on the post-it each time they read the book, this will help you keep track of how many times they are reading their "just right" books)
Here are some of the resources I am using to get started for "just right" reading...
Kimberly's Personal Sight Word Readers
Julie's "I See" Emergent Reader
Caitlin's Chicka Chicka Boom Boom Color Book and a simple ABC Reader
Julie's Brown Bear Color Word Reader
Deanna's Colorful Cats Emergent Reader
Colors Little Books Emergent Readers by Melissa Williams
Melissa's Foldable Alphabet Books
Kim's Cut a Sentence Unit