I wrote the wrong number on these books, then compounded the misteak by writing over it.
Looks awful!@!

I recalled learning about sanding sponges at SicPress.com. They sell book repair stuff, AND they have videos showing how to use them!
You merely scratch away at the boo-boo.

You want to support the edge of the text block so the pages don't tear. I used a book press. Not perfect, but good enough for the likes of me.

It's rather hard to get ALL of it off. That's why I used a Sharpie in the first place, so it would be hard!

After renumbering it, with a Sharpie pen, it looks pretty good. MUCH nicer than before.

You can hardly tell which books they were, eh?

The sanding sponge I used was a 3M product. I got the one with coarse grit on one side (and edge) , and fine grit on the other. It costs three bucks at the home improvement store.