I Rode a Horse of Milk White Jade, by Diane Lee Wilson (Orchard Books, 1998, middle grade/ya, 230 pages)
Oyuna lives in late 13th century Mongolia, moving across the steppes with her family and their horses. When she was an infant, bad luck befell her--her mother died, and her foot had been crushed by a horse. But she grows up determined to bring good fortune to her family--to win the great yearly horse race...even though girls aren't allowed to enter. First she needs a horse, a horse swifter than the wind...
But when her father gives her the chance to chose a horse of her own at the yearly gathering, fate leads her to an old white mare, and Oyuna is compelled to choose her. The bond between girl and mare is magical--Oyuna at times feels what her horse is feeling. When the soldiers of the great Kublai Kahn come to take men and horses off to the wars, and, against all odds, pick Oyuna's mare, she knows she can't let her horse leave without her. Disguising herself as a boy, she sets off on a magical journey across Mongolia that takes her the court of Kublai Kahn himself. There she might achieve her dream of a snow white mare who can win the great race...if she can change her luck. A much more detailed synopsis can be found at books4yourkids, where I first heard of this one.
This is the sort of book that manages to be both magical and believable--a once upon a time and far away story, whose detailed depiction of life and customs in medieval Mongolia make that time and place come to vivid life. And Oyuna is a wonderfully plucky heroine--brave and resourceful. There is magic here--Oyuna's mare is far from ordinary, and the spirit world lies close to what is real, but it isn't full blown fantasy. It's more historical fiction with a touch of the inexplicable.
An excellent book, in particular for the girl reader who likes horses! Note on age--this is catalogued in my library system as YA, but it had a much more middle grade feel for me. There's a romance toward the end, but it's a romance that just happens, and isn't part of the story--for the most part, is a Brave Adolescent on a Quest story, the sort of book that's very at home in the middle grade section!