I think the popularity of the books are directly tied into the HBO adaptations. It's not the first 'crappy novel' that became popular after being made into a movie or TV series. The Graduate, M*A*S*H, Forrest Gump, even The Maltese Falcon were novels that weren't widely read when first released. Winston Groom once said of Gump that it was just his luck that one of the 10 people to buy his book was a screenwriter.