Reference items for the setting of Ill Met By Gaslight:

The Era

• 1890 in the UK
• Victorian Era overview
• Queen Victoria
• Sherlockian.net - a repository of many links having to do with Sherlock Holmes' London.
• Map of Dickens' London

Society and Peerage

• Victorian High Society (original site, with more data, here)
• Peerage of the UK
• Peerage and Titles
• Forms of Address

Language

• Victorian Slang (lower class)
• American/Brit slang dictionaries (contemp.)

The Cops

• Special Branch
• New Scotland Yard (1890)

Whitechapel

• Jack the Ripper
• The Ripper and the Royals

Magical Stuff

• Order of the Golden Dawn
• Spring-Heeled Jack
• The Faerie Queene, by Spenser
• High and Low Church debates - How would these have differed if "magic" started working?
• Midsummer Night's Dream (full play)

Sundry Bits

• Coinage
• The Royal Baccarat Scandal
• Big Ben
• The Tower Bridge
• Drownings and Suicides, including "The Bridge of Sighs" (Waterloo)
• London Fogs
• Automobiles and Bicycles
• Opium use