This is a cute little 22 page story written by author Felicity Glum and illustrated by Lori Tyminski. It follows loosely/expands upon one of the story elements from the film.
What's cool about this book is that it's, as mentioned, "2 Books in 1" (in a way). Open it from the front side, seen above, and read the story from Joy's perspective. Open it from the opposite side, seen below, and read the same story from Sadness's perspective. What happens in the middle is clever!