I will tell you all straight off the bat: I never expected this to happen.

Once Upon a One Night Mistake (previously titled Once Upon a One Night Stand) was just a silly little story I started writing for fun—and escape—at 14 in 2007 on a website called FictionPress. It was my first foray into writing original fiction on the internet, an exercise in creativity for a teenager who really never thought she’d go anywhere with writing.

But more than 60 million reads later, that silly little sto...