The Infinite Moment of Us, by Lauren Myracle

If you liked Fault in our Stars or Forever (Judy Blume), you'll like this one. Definitely upper high school and up, in terms of appropriatness.

A fairly realistic exploration of first time love between two teenagers the summer before they go off to college, with some SIZZLING "romantic" scenes.

I find that YA novels often shy away from the reality of older teen relationships and this one doesn't.