[Angel Spirit Girl]

It is a series of juvenile (the level before YA) chapter books.

They are about a girl who is a princess in a fantasy world that she can cross over to with the help of a unicorn. This unicorn is sent from the realm of the unicorns to help her find her kidnapped royal parents and defeat the "evil baddie" who is trying to take over. I only read the first book. I think at the end of it, this kind of snooty girl, who is having a horse boarded at the princess's mild mannered secret identity earth horse farm, gets transported to fantasy Balinor.