K. M. Peyton is a wonderful author in the vein of Georgette Heyer, Jane Austen, D. E. Stevenson, etc. While this book takes some of the irritating liberties that modern romance writers tend to take with the Regency and Victorian periods, it is just good enough on the horse front to excuse itself. You come to really care about the heroine, who is just young enough to be your daughter and old enough to be yourself.
I would NOT give this to a girl under the age of 14 because of the sex, but would give it to any true horse lover that age or older. Another great(er) K. M. Peyton is "Right-hand Man," though I believe this is out of print. The Flambards series is, of course, also superb -- albeit a little later period.