As Terry Pratchett's been mentioned a hundred times already (my favourite author, particularly the Vimes-focussed Discworld books), I'll toss out a mention for Anno Dracula, written by Kim Newman. Basically, a trilogy of books set in a world where Dracula defeated Harker, Van Helsing and co. in the events that transpired in Dracula, and went on to marry Queen Victoria and turn a huge swathe of the population of England, men and women of every social strata, into vampires. The plot of the first book revolves around some nut, named Jack the Ripper by the press, murdering vampire prostitutes and the efforts of a few individuals to track him down. Subsequent books in the trilogy, the Bloody Red Baron (set in World War I) and The Judgement of Tears (Rome in the 1960s) look into the long-term effects on a world where vampires are everywhere. Really quite intelligent books, and (particularly the early ones) draw on a lot of public domain characters from works of Victorian fiction, as well as plenty of historical figures for all three. Not to mention that due to the benefits of vampirism there's aa good nummber of characters from the first book still kicking around in the third... even after the huge culling effect of the second book.
