I really enjoyed this. I thought the characters had wonderfully strong personalities. Elizabeth's feelings about her illness and her horse were written well. I could relate. Neither her death nor the possibility of her reincarnation as a horse were overly sappy or sentimental, as they easily could have been, and that was very nice.
Minor complaints: The girls did seem like they should be four or five years older than what we are told. For their voices and behavior, 18 years would be very believable. I assumed I was dealing with older teens until I read otherwise. I thought that having Rocket get lost and the stable's destruction was a bit too much, since neither event seemed to have any lasting consequences. I didn't understand Annie's need to lie to her parents about the situation in the first place, and since it also did not seem to effect the overall plot it seemed unnecessary.
That sounds rather critical, but I did really like it!