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.
You know what I'm talking about, iBook owners? The black marks from your hands that stain the white handrest area of the iBooks? How do you manage to clean that? Or do you just accept it and move on with life?