Me Before You by Jojo Moyes (2012) Pamela Dorman Books

Advertised as a Romance - but it is far more than that.  It's mainstream.  It's about living in a situation where none of us want to find ourselves.  It's tough, it's heartbreaking, it's redemptive like so few books are today.  This will stick around and sell year after year unless booksellers are heartless and illiterate.