This has been driving me mad. It's a science fiction book about a girl in her teens who appears to be the only survivor of a disaster. She keeps a diary addressed to "Unscheduled non-specific reader" and relates all her adventures is a slightly abbreviated style of writing. She appropriates an SUV, adapts it to her size and crosses America searching for other survivors. Eventually she finds some, but there are complications.

I have a vague feeling the author's name may start with H.