warriors, the guardian herd, riders f the realm, horse diaries, canterwood crest, umm some others that are YA(young adult) (Cleopatra Selene)

Umm...

Warriors, by Erin Hunter (series)

The Chronicles of Narnia, by C. S. Lewis (series)

War Horse, by Michael Morpurgo (stand alone book)

and a book I love, but is diffidently a young adult book is:

Ender's Game, by Orson Scott Card (has other books, but is not in an official series, and this is the best one)

Wings of Fire

Fablehaven

The Giver (quartet)

Harry Potter

Lord of the Rings

Janie Johnson Series

Warriors

(I'll comment again when I think of more!)