The Prince and The Queen, by Tiffany ReiszRead about another entry on the list.
Two books in one recommendation! Before reading Tiffany Reisz’ Original Sinners series, I had never read about a polyamorous relationship before, but that is the best description I have for what Nora, Soren and Kingsley share in this series. These two books are the ones that best explore how their love triangle: Kingsley and Nora both in love with—and submitting to—Soren, and Soren willing to do anything for both of them…except sacrificing the other. The history between these three characters is steeped in darkness, unfulfilled desires, and even, to a degree, loathing—but they are the most functional dysfunctional lovers that I love to read about, again and again.
Also see Ewan Morrison's top ten literary ménages à trois.