I maintain a Romance Scholarship Database. It's free to search and I hope it will be of use to people studying popular romance fiction. You can find a short introduction to the database here.
In Faith, Love, Hope and Popular Romance Fiction (available free, in full, on this website) 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."
With regards to those other definitions, the Romance Writers of America state that romance novels 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. I've written a short introduction to popular romance fiction, with a brief follow-up regarding the variety of sub-genres and countries of origin.
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.