I liked the Heartland series (the show, too! The show is more grown-up-y than the books), I read one book from the Phantom Stallion Series, King Of the Wind, Black Beauty, Born to Trot, Mustang, the Sonrise Farm Series, Fury-Stallion of Broken Wheel Ranch, and the Black Stallion Mystery. I'm sure I'm forgetting some. =]
I really like the show Heartland, which is loosly based off the books (it didn't end, but the books did.) The show is wayy better, though.
I pretty much read any horse chapter book, no matter what reading level it's supposed to be for. XD
I want to read My Friend Flicka.