The Unwritten Rule - Elizabeth Scott

Sarah and Brianna have always been friends, and it's always gone like this: guys talk to Sarah in order to get closer to Brianna. So even though Sarah met Ryan first, she's not surprised that he ends up with Brianna (even though Sarah has a massive crush on him). The three of them hang out, and Sarah and Ryan's friendship grows until one night an innocent exchange between them leads to a moment that makes Sarah realize that Ryan might be interested in her after all. But if there's one unwritten rule, it's this: you don't mess around with a friend's boyfriend. So Sarah tries to resist temptation. But with the three of them thrown together more and more, tension builds between Sarah and Ryan, and when they find themselves alone together at one point, they realize they just can't fight how they feel anymore....