A promo like this can push a book up the Amazon rankings. The more sales it gets, the higher it goes. The higher it goes, the more sales it gets, and so on, till it (hopefully) gathers momentum and becomes a best seller in its category (like #thememoryofwater did).
Once it’s up there it has the potential to hook brand new readers, which is really the ultimate goal.
#GreyMagic is currently sitting at #138 in Magical Realism and #37,836 overall in the Amazon store (out of over 11 million books). I’m going to run a few promos and try to get it up to #1 in Magical Realism. If it works, I’ll let you know!