The title of the book is interesting in the sense that it gives you a chance to guess who is Number 5. Is it an animal? A bird? The illustration of little children on the cover page and their expressions tell us that they are looking for something. The trees around them suggest that they are in a forest. It seems obvious that they must be searching for something that is their own, e.g., a pet animal.
This book is for beginner readers. Hence the storyline and the pictures both play an important role in connecting the reader to the story. If we view the story from this standpoint, it reveals the secret of who 5 is at the start. However, there is still ample scope to wonder where 5 is. Two of the characters go to different animals in search for 5. There are feathers in the illustrations that work as clues to help them in their quest.