If she liked Blind Beauty, she might like more of KM Peyton's books, like Dear Fred or Darkling.

My Friend Flicka by Mary O'Hara is an old favourite, though I suppose technically, it's a trilogy.

Dancer by Shelley Peterson is about a Canadian jumper.  It's interconnected with Peterson's other books - Abby Malone, Stagestruck, Sundancer - but standsalone, and is quite a lovely book (of Peterson's books, it's my favourite).  The main character is 16, but I don't think there's anything inappropriate for a 13 year old.  The main character of Abby Malone and Stagestruck is younger.

Robin McKinley's The Blue Sword and The Hero and the Crown, while not horse books per se, have horses in them as secondary and main characters, respectively.  She writes wonderful horses; they have personalities without being anything more or less than a horse.  Beauty also features a horse as a secondary character, though he has a little less to do than the horses in TBS or THatC.

Under A Different Sky by Deborah Savage is another great horse book. Though the main characters in this one are also a little older and some of the issues they face might be a bit mature for a 13 year old, although I think I first read this book around that age.  And if Blind Beauty is okay, I think the ones I've recommended would be too.

Willow King by Chris Platt was another one I enjoyed.  I didn't end up loving it quite as madly as McKinley's or Savage's books, but it was still a good read.  Shadow Horse by Alison Hart is another that falls under this category. I think I ended up wishing it was a series so I could read more about those characters, but I don't think it is one.

Marguerite Henry's horse books - Misty, King of the Wind, Justin Morgan, San Domigo, Born to Trot, etc, etc - are must reads, if your daughter hasn't already read them, that is.

Black Beauty, of course.

Man O' War by Walter Farley, though you may have already come across this one, if your daughter has read The Black Stallion series.

The Silver Brumby by Elyne Mitchell. This is a series, but it doesn't seem to be a very well known one, so I'm mentioning it. It's told from the horse's pov and quite well done, from what I remember. There's a movie called "The Silver Stallion" that's based on this book, starring Russell Crowe before he was famous; it's also one of the better horse movies out there, imo.

You might also find this website useful: http://www.ponydom.com/books/index.html

Thanks for the great list - her birthday is coming up soon so I will definitely look into getting some of these. 

Oh, I recently read "War Horse" by Michael Morpurgo.  It's the story of a farm horse that gets sent from Great Britain to France to fight during WWI....it's excellent! 

I loved all of Marguerite Henry's horse novels when I was a girl!

You've already gotten some really good suggestions, so I'll just add a couple that I've heard are good:

National Velvet by Enid Bagnold

Smoky the Cowhorse by Will James

i have several horse books on my shelf. i think some are series and some are not. i should have more from my little sister soon

Blind Beauty by KM Peyton.

I second My Friend Flicka, The Blue Sword, Beauty, and The Hero and the Crown... :)

Thanks everyone for all the great suggestions - I got her the Blue Sword and The Hero and the Crown for her birthday - they looked really interesting.   I am on the waitlist for the Silver Brumby and she's read the classics - National Velvet, Black Beauty , etc. plus a ton of others.  She just loves horses and it gets tricky finding new books for her.

The James Herriot(t?) books aren't precisely horse books, but I love horse books and I love them.  There's a lot of horses mentioned, as well as other animals.

I can't think of anything else that hasn't been mentioned.

There's also "To Race A Dream" by Deborah Savage.  I read it when I was younger, and it was pretty good.