Unbooked
Amy L. Wink's post reminded me that I haven't really culled my bookshelves in three or four years--in large part because, right now, there isn't much of anything I feel the need to toss. Usually, if I decide that I don't want a book occupying valuable shelf space, it's immediately after I finish reading it. A lot of my purely recreational reading, like SF or detective fiction, quickly disappears to eBay, Amazon, or my department's free book table. I treat composition textbooks and the like as ephemera, in essence, and tend to dispose of them on a regular basis. And books that are purely project-specific also have been known to disappear into exile--I haven't even bothered to enter the Anne Boleyn novels into my LibraryThing catalog.
If anything, I've more than once found myself in the odd position of having to re-purchase books I turfed out years ago. Very frustrating, not to mention uneconomical...