At the trashier end I used to inhale the Saddle Club books when I was 9 or so. And there are over 100 in the series. Probably will only appeal if they like horses though, as it's basically Jilly Cooper for girls who think horses are better than boys.
All girls think horses are better than boys, even if they don't have one!
I'm in waxing lyrical mood tonight as thinking about childhood books is taking me back to thinking about childhood. I loved animals (as my youngest does) and although we had cats, I yearned for a dog and a horse; ok a horse is out of bounds for most families, but I wanted a dog because of the loyalty, and being in the world of the Famous Five, I was George and NEEDED a dog.
Also, my childhood dreams consisted of living in Cornwall or Devon and on a farm, with the sea in smelling distance. So I've realised that as an adult, and we don't have a horse, but I have free access to exercising horses and we have a dog and... The beauty of having kids is that they just see normal as normal, and like us all, will probably yearn for something else in retrospect. Not that you don't appreciate what you have (in time) but somewhere along the line you lose your childhood, or imagination? Well I did.
QED, my brother doesn't like animals and his utopia is raising his kids in a city.