Something Queer in the Wild West by Elizabeth Levy, illustrated by Mordicai Gerstein, 1997.
Gwen and Jill go to visit Gwen’s Uncle Dale, who owns a ranch in New Mexico. The girls enjoy learning to ride horses, but Fletcher seems to be interested in an old barn. C. J., who works on the ranch, tells the girls that the barn is supposed to be haunted by the spirit of a wild horse and that they should stay away from it. Uncle Dale confirms the old legend, but the girls think there’s more to the story.
They have hear strange sounds coming from the old barn. Then, the girls notice that Fletcher seems to be sneaking out during the night. One night, they see a white horse roaming around when Uncle Dale doesn’t own a white horse. Could it be the spirit of the wild horse, or could it have something to do with a recent horse theft?
Fletcher’s odd behavior is a clue, and there is more to the old barn than the ghost story, as the girls suspected. The “theft” isn’t quite theft, and the thief is actually trying to do something noble. In the end, the girls decide to help, but they need Fletcher’s help to do it.