London is a d20 Modern-compatible supplement detailing London during the Victorian Era. From the publisher:

London is the greatest city of the Imperial Age. While much of the steel and production industries of England resided in Birmingham, the coal that drove the colonial enterprises was extracted in Wales, and shipping was increasingly centered in Southampton and Liverpool, London was the seat of power for the largest empire in the world. Millions walked its teeming streets, and the city has been the setting for diverse Victorian themes and genres, from detective fiction to steampunk.

The Imperial Age: London is a complete guide to using the city as a setting for your Victorian-era campaigns, no matter what the genre. Included are detailed descriptions of every district, informative sidebars on such diverse topics as money, precedence, Freemasons, workhouses, the Metropolitan Police Force and more, and suggestions on running adventures and campaigns in the greatest city of the age.

Included within the pages of this supplement is a genuine Imperial Age document -- the 1899 edition of Bartholomew's Pocket Atlas & Guide to London, featuring detailed street-level maps of every corner of the city, as well as a reference index to streets and places of interest.