Johnny Tremain, by Esther Forbes, is the story of a prideful boy who suffers a great loss but learns how to be a better and more productive person because of it.
When Johnny can no longer function as a silversmith apprentice because of an injury to his hand, he is forced to find another way to provide for himself and earn a living. It is quite difficult, at first, because he believes he is better than any job people seem to think he is qualified to do. Finally, desperate for money, he accepts a job working for the Boston Observer, a job he had earlier considered to be beneath him.
To deliver the paper, he must learn how to ride a horse, something which comes quite naturally to him and is not impacted by the injury to his hand. To earn extra money, he delivers letters, including "messages to sympathizers of the Sons of Liberty," as you mention. With this extra money, Johnny buys himself "second-hand spurs, boots, and a fur-lined coat."
The Revolutionary War is approaching, and both his job and the equipment he buys will help him do his part to fight the British. When someone asks sixteen-year-old Johnny if he is a boy or a man, he laughingly answers: "A boy in time of peace and a man in time of war.” His ability to ride a horse well (and even better because of his new gear) is a gift which will be used in this "time of war."