What story book you pick will depend on the child (or children) you are making it for.
If the story sack is for a specific child, you might want to take into account any special interests of favorite characters that child likes. For example, you might not want to pick out a story about cats if the child loves dogs! The more interested the child is about the subject of the story, the more likely they will be to read and truly enjoy the book. You should also pick a book appropriate for the child's age and abilities.
Visit your local library to see what is available, or do some research on the Web to see what books are popular right now!
Once you have decided on a story you need to fill your sack with related items...