Misty of Chincoteague, by Marguerite Henry

Oh, Marguerite Henry, I have been twice proven wrong.  What on earth did I have against you and your horse books when I was little?  These are really solid, fun, appealing books.  I stand corrected.

Based on a true story, this book follows Paul and Maureen, two siblings who live on Assateague Island off the coast of Virginia.  Wild horses live on neighboring (and unsettled) Chincoteague Island, and every year the people on Assateague Island have an event where the men round up as many horses as they can and swim them across, so they can be penned and sold (the proceeds benefiting the fire department).  The most desired horse is the Phantom, a mare who is so fast and unruly that she has never been caught.  Because this is such a tradition, the townspeople also have events and raffles and a big community dinner.  This year will be Paul's first assisting with the roundup, and he is sure that he will be able to catch Phantom.  Not only that, but he and Maureen would like to own her.  They spend the summer digging clams and performing odd jobs for the neighbors, until they've collected a full $100.

On the day of the roundup, Paul finds himself left behind on Chincoteague Island while the more experienced riders race ahead without him.  Alone in the trees, he comes across Phantom who, to his astonishment, is with a colt and allows him to round them up.  During the swim, the colt begins to sink because he is too young to swim the distance alone, but Paul jumps from the boat and swims with him, holding his head out of the water.  Paul and Maureen realize that they will have to buy both Phantom and the colt, who Paul names Misty.  They plan to get to the pony sale the first day to ask the fire chief to reserve the two horses for them, leaving their $100 as a deposit and planning to earn the remainder through the rest of the Summer.

Due to circumstances outside their control, Paul and Maureen are unable to reach the sale in time and they find that Misty has already been sold.  Fortunately, a turn of events works out in their favor and she eventually does become theirs.  The remainder of the book concerns their raising her and teaching her to love her new home.

Misty eventually became the author's own horse, and lived with her in Illinois.  Although many of the events in the novel are stretched or outright fictionalized, it paints a picture of the region and the origin of the horses that bear the island's name.