It’s hard to go wrong with anything written by Marguerite Henry! Check her Amazon author page for more titles, I just saw a couple there I missed reading as a kid. King of the Wind and Justin Morgan Had a Horse are wonderful. She wrote so many truly great horse books and many are based on real horses or real events.
Here are some picture books for the younger crowd:
A Field of Horses (Peter Hansard) The best book with basic horse care info for littles without being boring.
Fritz and the Beautiful Horses (Jan Brett) sweet story about who you are being more important than your appearance.
Black Beauty (Adapted by Sharon Lerner, illustrated by Susan Jeffers) I’m saving Black Beauty to read aloud in when my own horse-crazy 4 yo is ready for the richer language. But she loves this picture book tremendously!
My Chincoteague Pony (Susan Jeffers) Since she is ready Misty rignt now, she would probably like it. It’s the story of a girl who saves her money to buy her own pony from the pony-penning days. If my memory serves me correctly there is a letter from Susan Jeffers to Marguerite Henry included at the beginning. Susan Jeffers also did My Pony, based on her childhood. Even if it’s only for the pictures they are nice because Susan Jeffer’s horses are lovely.
Billy & Blaze series (CW Anderson) He also wrote some others such as,The Blind Connemera, but I’m not familiar with his other books, I’ve only read the Billy and Blaze ones. If she’s reading Misty she’s past the reading level in Billy & Blaze, but I thought I’d list them anyway.
Star of the Wild Horse Canyon and Take Care of Dexter (Clyde Robert Bulla) haven’t read these yet, but he’s a good author, and they are on my book wishlist.
While I read Walter Farley’s Black Stallion series as a girl, I think some were better than others. Houseofchaos mentioned the two I liked the best. Also, I don’t know how many people know about it, but Water Farley wrote an outstanding book about Man O’ War.
Of course, don’t miss Black Beauty (this edition has nice artwork) and go for an unabridged verison when she’s ready. I own 3 copies of Black Beauty including my grandpa’s from his boyhood days!
The Blind Colt (Glen Rounds) there are wolves in the story, I *think* it ends okay, but might be one for when she’s older.
The Summer Pony (Jean Slaughter Doty) not nearly as good as Margurite Henry, but better than the Saddle Club type books!
Also look at the Kate Seredy books when she’s older, the Chestry Oak is AMAZING it’s really about WWII but it has horses as a part of the story. There was just a thread here about it recently. I haven’t read The Good Master or any of her other books yet.
Also, if your family decides to pursue horseback riding, the United States Pony Club Basics for Beginners Level D by Susan Harris (book one in a series) a very read-able “how to” manual about horsemanship. Wether or not she chooses to ride English, there is a lot of valuable information about safety and horse care here. I happen to have the older edition haven’t seen the second one yet. But imagine it’s good. My old copy of The Usbourne Guide to Riding & Pony Care has lots of drawings, the newer edition might have photos. (I used to read horse care books for fun). I also just saw, Cherry Hill’s Horse Care for Kids on Amazon, I’ve not read it, since it was published after I quit teaching riding and had kids of my own, but it looks like it might also be helpful.
Off to finish getting ready for an upcoming garage sale, it was fun looking at all my childhood horse books and on Amazon for ones I missed reading as a girl. Hope my long-winded list helps!