Kindred by Octavia E. Butler

In 1976, Dana dreams of being a writer. In 1815, she is assumed a slave.

When Dana first meets Rufus on a Maryland plantation, he's drowning. She saves his life - and it will happen again and again.

Neither of them understands his power to summon her whenever his life is threatened, nor the significance of the ties that bind them.

And each time Dana saves him, the more aware she is that her own life might be over before it's even begun.