Some of the most detailed mapping of London by Ordnance Survey from just over a century ago is now available online through National Library of Scotland.
The London "five feet to the mile" series covered the capital in 729 sheets, based on a revision and survey of 1891-1895. These maps provide excellent detail of the whole urban infrastructure, including residential and industrial premises, schools, asylums, hospitals, parks, canals, docks and railways, and even the interior layout of public buildings, such as cathedrals, churches, and railway stations.
The maps have been georeferenced so they can be viewed as a seamless layer on top of present day maps and satellite images.
http://maps.nls.uk/os/london-1890s/