This book consists of a variety of letters from many crayons letting Duncan know why they are upset. They all give very good reasons, and it is quite humorous. At the end, Duncan uses all of the colors to make the crayons happy again.
I would use this in lower grades, most likely first or second. I would first show this book to the students and ask them why they think the crayons want to quit. After reading the book, I could ask them if they have any crayons they think might want to quit (from too much use or from not enough, I'm sure they could come up with something). Then, they could write a letter to the crayons explaining why they shouldn't quit, or they could pretend they are a crayon writing to their friend about why they are upset. Either one would be a good creative writing opportunity. They may even use their crayons better.