I thought about one of those, but I don't have a rail for it, and my library doesn't have the right configuration anyhow. I've settled for two book carts (one of them the kind that lets you stack books in a corner and move them from one place to another (which came in handy a few months back when we moved 2 bookcases around), the other [which I'm not actually sure where it will live] a 4-shelf double-sided cart).