Anno Dracula by Kim Newman
Many years ago (so many, in fact, that I believe I may have referenced it in the dissertation on the character of Dracula that I wrote for my degree) I read a short story by Kim Newman in a vampire-themed anthology which proposed that Count Dracula, instead of being destroyed by Van Helsing, could have married the widowed Queen Victoria and become the second Prince Consort, spreading a plague of vampirism through the nation.
This story was then developed into a series of full-length novels, the first of which, Anno Dracula, my brother bought me for Christmas. Anno Dracula expands on the above premise, taking as its setting a version of Victorian London in which vampirism is rife (and even fashionable) and the Prince Consort has brought back impalement as a form of punishment. Amid this steampunk-gothic dystopia, secret agent Charles Beauregard is assigned by the mysterious Diogenes Club to solve the mystery of Jack the Ripper, who brutally dismembers vampire prostitutes with a silver knife. He's aided by Geneviève Dieudonné, a vampire 'elder' from mediaeval France who pre-dates even Dracula himself.
Anno Dracula is full of period atmosphere, whether we're meeting the characters in a dark Whitechapel alley, a disreputable pub or a fashionable society drawing room. Newman also appears to have put a huge amount of thought into every conceivable consequence of vampire rule which combined with the well-researched Victoriana makes for impressive world-building. Beauregard and Dieudonné make an engaging pair, and Newman also makes a excellent job of developing a number of characters created by other writers, including Bram Stoker's John Seward and Arthur Holmwood and John Polidori's Byronic vampire Lord Ruthven (now Prime Minister).
The plot proceeds at a fairly steady pace and builds to an adventurous climax, although in some ways the plot itself is somewhat secondary to other elements of the book. Kim Newman is not only a prolific writer of fiction but also a renowned academic expert in all things horror, science-fiction and fantasy - and it shows. If you happen to have an interest in vampire myths, Victorian literature or horror cinema, the sheer number of mentions of familiar characters and events will make you dizzy; barely a page goes by without one and the character notes at the back of the book run to sixteen pages.
There is a degree to which this can distract from the story itself. Although I found it great fun because I share these interests and my frame of reference for this kind of thing is huge (my brother enjoyed this aspect too and gave me the book for this reason) I would imagine that someone less obsessed would miss out on a lot of the in-jokes and nods towards other sources, which are often included seemingly for their own sake. In other words, you don't have to be a hardcore Dracula aficionado and vintage horror fan to read this book ... but it helps.