The Money Creek Mare was published in 1981 and is long out of print, but if you can find a copy, it's worth the trouble. It's the best sort of horse story: the horses are important to the plot, but the main character's life and relationships are just as vital.
Fifteen-year-old Ella Rae Carmody has been trying to hold her family together since her mother left to pursue her Hollywood dreams. Ella Rae believes in her father's dream of owning a racing stable to rival their rich neighbors', the Puckett-Smythes, even if her father doesn't seem to be capable of much more than dreaming. He brings home a crippled mare won in a card game, but it's obvious to Ella Rae that the horse will never race again. Then again, she might make a good brood mare--and the Puckett-Smythes have a world-famous racing stallion in a paddock where anyone could get to him....
The setup is good by itself, but the book takes some surprising turns. Ella Rae takes a job as a housekeeper at the Puckett-Smythes', mostly so she can find out more about where the stallion is kept and how to safely steal him for a night. The money comes in useful too, since the diner Ella Rae's family owns is deeply in debt. But before long, reserved and lonely Mrs. Puckett-Smythe offers her an easier life: be adopted into their family and raised as their own daughter. In exchange, they won't prosecute her for horse rustling and won't charge her the stud fee for their stallion's services.
The real focus of the book is Ella Rae's struggle to discover where she really belongs. She resents her father's new girlfriend, who steps in and takes over when Ella Rae is convinced the family was getting along just fine. She worries about her dreamy and impractical father, worries about her two younger siblings. When she accepts the Puckett-Smythes' offer, she's thrust into an affluent world where she knows she doesn't belong--and is sent off to a girls' boarding school to learn how to fit in.
It's a fascinating plot, and Ella Rae is sturdy, clever, loyal, and likable. She makes hard decisions because she knows what needs to be done for the people she cares for, but she tends to leave herself out. Her journey to discover where she truly belongs is poignant but very funny.
The horses don't play a big role in this story, but I loved it when I was younger and crazy about horse books.