Interesting that the likes of Ray Bradbury and Kurt Vonnegut still appear to be selling better than the likes of David Weber. Not entirely sure how believable that they alone amongst the deceased greats of SF should be in the top 20.
Well, it means they're selling better on Amazon but not necessarily overall. I doubt they are as easy to find in actual shops as someone like David Webber is, so the Amazon sales could be pretty much all of their sales. They certainly aren't all of Webber's, by a large margin.