I think some of you are mixing up the meaning of category versus single title and genre versus literary. Nora Roberts writes what is classed as genre fiction. She started out writing category romance for Harlequin but then became a writer of single title romance in the sub genre of romantic suspense writer which is a sub genre of romance. When people talk about "genre fiction" they generally mean not literary fiction.
Nora Roberts is not the best seller of genre fiction many writers in the thriller genre outrank her as well as horror writer Stephen King, fantasy writer JK Rowling and a number of others.