I really loved the Ranger's Apprentice series. I recommend it all the time for lovers of adventure stories. I enjoy how the characters have to use skill and wit to defeat their enemies in a fantasy world that doesn't have magic in it. It also has quite a bit of humor in it without being silly.

"The War that Saved My Life" by Kimberly Bradley is an excellent horse book, "War Horse" by Morpurgo, and of course classics like "National Velvet," "Misty of Chincoteage," and "The Black Stallion."