The Host (2008) by Stephenie Meyer
In the aftermath of the Twilight series I fear The Host is overlooked or dismissed by everyone who was sick to the bone of Stephenie Meyer and her endless love-triangles. So it is with a touch of irony that I say that this book is an excellent work of science-fiction-esque romance, centering on a love triangle. Meyer seems to have started with the thought: what would happen if there were two people inside one body and one of those people was deeply in love with another person? Or, what would a love triangle look like with two bodies and three personalities? The alien invasion that enables this situation is almost a side-issue, but is a fascinating what-if exercise in the vein of John Wyndham. Specifically: what would happen if aliens invaded earth and did a better job of being people than people do? Really, I felt that the romance got in the way of it being a good science fiction novel, but then romance readers are probably frustrated by the science fiction elements getting in the way of a good love-triangle, so I guess no one wins.