Lady Grace & the War for a New World (Earth’s End) (Volume 2) by Sandy Nathan

Not everyone dies in the nuclear conflagration. Surviving any way they can, the lucky ones make their way to Piermont Manor, Jeremy Edgarton’s ancestral home. Jeremy is the sixteen-year-old tech genius who might have stopped the atomic war, but couldn’t. No one knows how much time has passed since the world blew up.