From the back of the book:
Cthulhu and his minions, in the 1890s sharing the globe with the might British Empire, had duties to an empire of their own: a dark and cruel design against the ownership of the world and the dreams of humanity. Even in the peaceful fields of rural England only intelligent and energetic intervention could keep the shadows at bay.
Cthulhu By Gaslight includes a lengthy roleplaying adventure, "The Yorkshire Horrors" in which investigators join forces with the world's most famous consulting detective, Sherlock Holmes! Extensive background essays provide period skills, social classes, world politics, biographies and timelines for the 1890s, maps and London location notes (including the best stores of the time), travel, criminals and police, Cockney slang, cost of living, royalty and titles, club life in London, the occult in the 1890s, prices, and clothing. A lengthy essay considers time-travel rationales for moving investigators of another time in the 1890s.
The differences between this and the first edition include, but are not limited to: new essays, six full-color plates, additional maps, and revisions and corrections.