I recognize "A Court of Thorns and Roses" because my wife likes to complain about it. To hear her tell it, they're middling fantasy novels that are popular because they include graphic sex, but otherwise have a stereotypical plot and flat main heroine who is nevertheless irresistible to all the male characters. Sounds similar to how "Fifty Shades of Grey" captured the attention of an audience who wasn't previously familiar with that sort of content.