Paint the Wind, by Pam Munoz Ryan

After her parents died, Maya lived in near total isolation in her grandmother's house.  But when her grandmother dies, Maya is sent to live with her mother's people in Wyoming, of whom she knows nothing.

The two environments couldn't be any more different.  At her grandmother's, she was forbidden from any sort of rough play, taught to fear nature, and raised to despise her mother and idolize her father.  But at her new home on the range, she is thrust into adventure monitoring wild horses and sleeping in dusty tents miles from civilization.

At her new home, she learns to ride horses (a lifelong dream) and discovers a love for watching wild horses.  One of those wild horses (Artemisia) was once tamed by Maya's mother.  Now roaming free, Artemisia is in danger and Maya is anxious to rescue her (an through doing so establish a link with her Mom).  But finding the horse will put Maya in mortal danger.

Classic horse story material.  A few predictable tropes (dead parents, friendly wild horse, etc.) but overall the story is thrilling and loaded with horse facts and trivia.  Having not read a lot in the genre before, I was surprised at how explicit it is, but that's almost certainly part of the appeal (and probably why boys don't tend to go for these stories!).  Some of the action at the end was rushed and compressed, in sharp contrast to the gentle pace at the beginning of the story.  Ryan seems to enjoy exposition more than conclusions, and pushes through the endings as fast as possible.