Did you actually try heating the glue back up before you scraped it, or did you just go at it cold? Because hot glue that drips and then cools on a book cover is a whole different beast than the sticky mess you get when it's still warm. I've made that mistake before, thinking a butter knife would do the job, and I just ended up smearing it into the fibers. If you haven't already, hit it with a hairdryer on low for a minute and see if it softens enough to peel off in one piece, that might save you the paint route. But if it's truly fused into the cover, painting over a whole vintage atlas is a shame, but sometimes you gotta do what you gotta do to make it presentable.