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That depends on the kind of eBook that your mom listens to. Looking at my own playlist, I have lengthy ones like The Wheel of Time book 1 at 660 MB, while short magazine-type ones like an issue of The Economist at 180 MB.
The thing with eBooks as well is that once you finish a book, you normally don't listen to it again anytime soon (or ever) unlike songs that you can keep in your iPod and listen to repeatedly. Most likely she'll remove the older eBooks that she finished already, and thus will not need to keep them sitting around in her iPod taking up space.
If your mom doesn't plan on stuffing it with thousands of songs, I'd say 8 GB is more than enough.