In the past we have used a bit of carpet offcut or a piece of carpet underlay. Both do the job beautifully but look bad. A folded piece of pretty cloth (or cloth wrapped around carpet underlay) would work, but before long it will be dirty.
Trouble is, it is quite a huge job to empty the bookcase completely and lift up the edge in order to replace/wash whatever I have put under the front edge. So that doesn't really happen! I've just done it for the first time in years.
I think I need something functional-looking which is softish and doesn't fray or look bad when it gets some dust on it. The flooring is laminate, and having something non-scrapy is essential so I can slide the bookcase out and clean behind it or retrieve lost objects occasionally.
Cheers!