After London, by the Victorian era nature writer Richard Jefferies. It was one of THE first science fiction books...even before some of HG Wells' classics. The story of London (and Great Britain) after some kind of worldwide disaster...natural or manmade. Jefferies also wrote his remarkable mystic autobiography, The Story of My Heart, shortly before his death in the late 1880s.
Like many other now public domain works, it's available here at Project Gutenberg:
http://www.gutenberg.org/etext/13944
The Victorian era writing takes some getting used to, but stick with it.