How would you charge if you're tasked to do a book design (images and text provided) that will be mass produced for sale. Or anything that will be mass produced in general. Do you charge a flat fee or a percentage?
This is strictly my opinion.. take it or leave it.
For me, it would depend on how well I thought the item, in this case the book, would sell, and the type of audience it was geared towards.
I would consider if it would be well marketed, AND if it was something people would be interested in buying. If the product has longevity, a percentage might net you a substantial profit, but probably over the course of many years, especially in the case of a book, depending of course, on the content and popularity.
If I wasn't convinced of those things. If I wasn't sure that it would bring me some sort of profit, year after year...personally, I would charge a flat fee. I'd rather kick myself but be able to have the credit for designing the unexpected blockbuster, then grow old waiting to see a percentage from my beautiful design on a bomb.