If you'd like to read a brief introduction to popular romance fiction, I've got a couple of posts up at the University of Birmingham's Popular Literature blog:
Part 1: https://blog.bham.ac.uk/poplit/an-introduction-to-popular-romance-1-by-laura-vivanco/ (defining the genre)
Part 2: https://blog.bham.ac.uk/poplit/an-introduction-to-popular-romance-2-by-laura-vivanco/ (variations + different subgenres)