A teenage girl uses a time machine to visit her teenage grandmother.

As the two teenagers look identical, they decide to swap places so that:

- the granddaughter can see how her grandmother lived;

- the grandmother can see what the future looks like.

In the future, the teenage grandmother sees herself in her old age, and is mortified to discover that her face is disfigured. This is due to a car accident she has as a teenager, an accident that her teenage granddaughter is at risk of having.

She then makes efforts to save her granddaughter's face by traveling back to the past.

Who wrote this book? What was it called?

I forgot to mention, the era of the grandmother as a teenager is our current era (the present). The era of her granddaughter is the future (with all sorts of technological changes).

So no, it's not “The Mirror”