I have a huge book collection, mainly things for which there is no adequate digital substitute (e.g. childhood favorite picture books, obscure books about women's labor history during WWII, a copy of Crime and Punishment with my dad's class notes scrawled in the margins). They are on my built-in bookshelves in the kids' playroom, along with the board games and our meager collection of DVDs. (Except for collector's edition box sets, we chuck the cases and keep them in sleeves in a media box. I think we have maybe 2 dozen grown-up movies; the other 100 or so are for the kids, and most of them were given to us by my brother when his kids outgrew them.)
I had a bunch of those little "gift books" that my Mom has picked up for me over the years. I threw them away. I didn't even bother taking them to a used bookstore or donation center -- they're just trash. (Crazy About My Daughter, which I just recently unearthed, was clearly targeted at teens. I got it for my 30th birthday. Seriously, Mom, did you even open it before you whipped out your credit card? The bit about spending hours in the bathroom primping wasn't even true fifteen years ago.)
Unlike a lot of people, though, I have actually read most of the books on my shelves. Many of them were school assignments, and a lot of them are children's or YA books, but I don't buy books I don't intend to read just because I want to give the impression that I'm the kind of person who would. If you pull a book of my shelf and ask me about it, odds are good that if I haven't read it, Pedro has. Still, I have a lot of books that I've bought and never read, or that were given to me and I never got round to, and every time I run across one, the ghost of Thomas Jefferson appears and gives me a stern look.
I hate that there are books on my shelves that I haven't read. It makes me feel pretentious, even though I do mean to read them all someday. But I wouldn't say they're purely decoration. For one, they're not in a part of my house that a casual visitor would see. For another, they're crammed on the shelves every which way to make them all fit, so they're actually not very decorative at all. Not the way you seen in Pottery Barn catalogs and the like. For a third, a lot of them aren't the kind of books you display if you're trying to look smart. A lot of them are graphic novels and geeky fantasy literature. (Most of the FanLit is Pedro's, actually. Mine are the sci fi and mystery paperbacks.) Not exactly an impressive collection of classical literature.