What a delightful story about a young orphan girl traveling west with a wagon train! Again, a child is paired with a stallion but it actually works in this story. (She doesn't ride it)
Katie Rose lost her immediate family (in Book One I guess) and wants to travel to Oregon to find her relatives. She will walk the 1,000 miles on foot, leading her Mustang - how she happened to have this horse must also be in Book One.
She has quite an adventure as she begins her journey and this Book Two only covers the start of the trip through crossing the Missouri River.
The HOOFBEATS series has other authors, but the four books by Kathleen Duey are all about Katie and the Mustang. The descriptions of the obstacles - rain, mud, stone-throwing boys and even fire - are unforgettable. I especially liked how well Duey explains the horse's reactions, both from the viewpoint of Katie and the Mustang.
This is a "must read" for horse lovers of all ages, but I suggest you start with Book One! I plan to find the other three books about Katie and the Mustang!
The author's site is http://www.kathleenduey.com/ and there is a page about Katie and the Mustang Books on the Hoofbeats Page.