First, a warning. If you plan on reading Wilkie Collins's works, particularly The Moonstone, you may want to wait before reading this as it gives away the plots and culprits. I bought The Moonstone a few years ago after learning that Collins was the progenitor of the detective novel, and while I'm sure I'll still be able to enjoy it, it will lack mystery.
As for Drood, I happen to love unreliable, less-than-nice yet somewhat charismatic first person narrators, and Collins makes for a good one. His relationship with Charles Dickens is the epitome of what can only be called "frenemies," and it's only made worse by the fact that both are writers. I don't read a lot of historical fiction, but I learned a lot about London at the time, and about Dickens and Collins, in non-didactic fashion. Most of all, you never know what is real or mesmerism or a drug-induced phantasm on the part of Collins (he was a laudanum and opium addict), but it all makes for a crazy, gruesome, creepy story.