The Unwritten Rule by Elizabeth Scott
Everyone knows the unwritten rule: You don’t like your best friend’s boyfriend. Sarah knows that and she doesn’t want to like Ryan but she just can’t help it. She’s liked him forever. Except now Ryan is dating Sarah’s beautiful best friend Brianna and Sarah must accept their relationship and forget about him. But something happens one night that makes Sarah question everything from her loyalty and friendship with Brianna to the depths of her feelings for Ryan.