Apple should just include a package of Mr. Clean Magic Erasers with all iBooks sold. 😛
Anyways, you could also try a bit of rubbing alcohol and a cotton swab. Just be sure not to swab the trackpad.
Edit: And no, oh dear God don't use a normal eraser (Well I suppose you could use it for the handrests but don't use it for the outer case!)! I scratched my baby's outer case up pretty bad using a normal white eraser. 🙁