Before I read spanking stories, I was always thrilled to find a spanking scene in a romance novels. There are two I still remember, and its been years since I discovered them.
In ONCE A PRINCESS by Johanna Lindsay, the hero, angry at the heroine, starts to give her a spanking, then the mark (birthmark, tattoo, mole? that I don't remember) on her bottom tells him she's the princess he's been sent to find.
In LADY BE GOOD by Susan Elizabeth Phillips, the hero, who is on the geeky side, has had enough and gives the heroine a spanking.
I reread both these scenes many times. Anyone else have a mainstream spanking scene they like? Here's an extensive list of possibilities from CF Publications. http://www.cfpub.com/indexstories/bibliography.html