I've got most of the Black Stallion books, as well as Marguerite Henry's books (all hard cover). I also have most of Albert Payson Terhune's collie books, and I tend to collect illustrators - Paul Brown, CW Anderson, Wesley Dennis. I look for the hard cover editions of favorite books, wherever I can find them. St. Louis has a pretty big book fair every spring - runs four or five days, and prices are pretty reasonable.
We have book cases all over the house. My book interests vary - I've got a nice collection of horse and dog books (mostly reference - training, breeds, health, photography, psychology, etc., as well as novels); reference/regional books on flora and fauna (I like to identify whatever I'm taking photos of, whether it's birds, butterflies, dragonflies, wildflowers, shells, etc); biographies, science fiction, murder mysteries, historical novels, regional history, poetry, history (Sis is a Civil War buff) and the original novels that inspired favorite movies.
I tend to cycle through a few series (Horatio Hornblower, James Herriot, Harry Potter, Dragonriders of Pern, Lord of the Rings) every five/ten years or so. They're old friends...