This is a spoiler, short and to the point. I watched "Swimming Pool" late-night, when I'm not at the top of my game figuring out movies with a twist. I did find note and troubling the appearance of the Brit "Julia" at the end of the movie vs French "Julie" who appeared throughout. It wasn't until I related the movie during breakfast the next morning that the most plausible explanation dawned on me: the movie told the tale of the book presented to Sarah's publisher. The last few seconds hint strongly at this explanation.