The Rachel Incident by Caroline O'Donoghue

The Rachel Incident is an all-consuming love story. But it's not the one you're expecting. It's unconventional and messy. It's young and foolish. It's about losing and finding yourself. But it is always about love.
When Rachel falls in love with her married professor, Dr Byrne, her best friend James helps her devise a plan to seduce him. But what begins as a harmless crush soon pushes their friendship to its limits. Over the course of a year they will find their lives ever more entwined with the Byrnes' and be faced with impossible choices and a lie that can't be taken back...