Follow your dreams.
It's a sweet book. Basically a little boy named Ben loves to listen to jazz music at a club near his house, especially the trumpet player, and he likes to play air trumpet and pretend he has a trumpet in his hands. One time he's sitting there playing, and the trumpetist from the club walks by and compliments his horn. He's so happy! But later, some boys catch him playing pretend horn and a bunch of kids make fun of him so he stops, and the trumpeter sees him again and asks, "Where's your horn?" Ben replies, "I don't have one." The trumpeter says, "Come on over to the club, and we'll see what we can do."
Basically it's got a pretty strong, but not anvilicious message of "don't let people get to you." All you need is one person to help you with your dream. Ben really loves the trumpet and music, and it's great that an adult sees that in him and takes the time to nurture his desire.