That I'm not very grown-up? I've only very recently (at age 24) ventured into the 'grown-up' section at the bookstore, instead of shopping in the teen sections. Mostly because there are so many books in the grown-up section and I get nervous. So, basically, my bookshelf is my collection of Royal Diaries, Dear America and Dear Canada diaries (all of which are mostly there because they're really, really pretty), most of the Princess Diaries books, Richard Scrimgeour, a few miscellaneous books I saved from my teen years, a complete Harry Potter set and the three 'classic' books I own, The Three Musketeers, Around the World in 80 Days and The Scarlet Pimpernel. Even the classics, though unabridged, are from the kid's section of the store. Though The Pimpernel cover is pretty awesome.
Then there are my 'grown-upish' books. A very spotty collection of Nero Wolfe books, a compete Codex Alera and Dresden Files (minus the books out working the cause).
Those are my display books, in the bookshelf that is visible. The one in my closet contains all my geeky collector books, behind the scenes movie stuff and biographies, a few baby name books, plus some Shakespeare (including a very, very beat up version of Romeo and Juliet well-loved during the process of writing a parody for drama club), a couple of Sophie Kinsellas and books I have bought that are waiting to be read.
Most books I don't keep, so it's not a very great reflection on my book choice. I usually read and then donate to Value Village, or pass them on to friends.