I'm currently working on a book about Faith, Love, Hope and Popular Romance Fiction and in it I suggest a new definition of the romance novel to complement other definitions which focus on structural elements: "modern popular romances are novels whose authors have assumed pastoral roles, offering hope to their readers through works which propagate faith in the goodness and durability of love." Popular romance novels, as defined by the Romance Writers of America, contain 'two basic elements': 'a central love story and an emotionally satisfying and optimistic ending'. In other words, these are happy stories of (eventually) requited love, though there may be some suffering and many obstacles to be overcome before the lovers reach their happy ending. A slightly more detailed introduction to popular romance fiction can be found here, with a brief follow-up regarding the variety of sub-genres and countries of origin.
The first half of Faith, Love, Hope and Popular Romance Fiction is now available for free on this website. I've written two other books about popular romance fiction: Pursuing Happiness: Reading American Romance as Political Fiction (2016) and For Love and Money: The Literary Art of the Harlequin Mills & Boon Romance (2011). I have also written a number of articles about popular romance fiction. I have my own blog on this site and, in addition, an index is available of many of the topics and romance novels I have analysed on the Teach Me Tonight academic romance blog.
I also maintain a Romance Scholarship Database. It's free to search and I hope it will be of use to people studying popular romance fiction.