I read a book at GCSE a few years ago about a girl who lives in a valley when a nuclear bomb explodes and destroys all surrounding life, except her in the valley. At least, she thinks so, until a man appears in a radiation proof suit. It explores how the only two surviving people get on with each other in the valley (not very well, as it turns out!). It was such a good book, but I simply can't remember what it was called! Please help!
Just to tidy up the spelling - yes it's 'Z for Zachariah' by the late Robert C. O'Brien, which won the Edgar Allan Poe Award. There's an interesting review of the book here: http://www.lostbooks.org/reviews/2002-01-07-1.html
and it's available to buy in the UK (as is the York Notes GCSE crib sheet guide!!)