I'm very sorry to discover that this is the last unread Brandreth "Oscar Wilde" mystery left for me. This one takes advantage of the fact that Wilde and Bram Stoker overlapped not just in place (both were Irish and went to university there together; Stoker was actor-manager Henry Irving's factotum at the Lyceum Theatre) but in personal lives (Wilde was an early suitor of Florence Bascombe, Stoker's wife). Furthermore, Stoker was distantly related to Arthur Conan Doyle, a major figure in these Wilde novels. It was inevitable that he would make an appearance in this series, even though the novel Dracula was published in 1897, too late for it to be directly referenced in any story of Wilde at large and at his best in London. Instead, Stoker's connections are used to take us into an underground (and not very serious) secret society playing with supernatural and vampiric rituals - no doubt thoroughly researched, as usual.
Other threads of the '90s that Brandreth manages to weave in here are the early investigations and experiments into "hysteria" - female mental illness - in London and Paris; and the notoriety of the Prince of Wales' son, Prince Eddy, who was the subject of (discredited) rumour that he was Jack the Ripper. He is planted here as a character partly to provide a red herring; his royal father also plays a fairly crucial part in the plot, in that he and the dignity of the royal house are the reason why Wilde & Conan Doyle's investigations are both commissioned and then suppressed secretly. The Duchess whose murder precipitates the whole thing is, of course, fictional, but the name and the situation are realistic enough that you have to confirm that with a little external Sherlocking on your own.
I really liked the multi-layered narrative here; since the not-so-underlying theme was female sexuality (and violence against it), the story being told through a sequence of letters, telegrams and diaries, mostly addressed to or directly referencing Mrs. Wilde, Mrs. Conan Doyle, and Mrs. Stoker, gave the narrative a welcome context and complexity.
And Wilde's relationship with the young man who at least put himself forward as a genuine vampire was, to say the least, interesting...