Always Second Best / Driven by Envy / Green-Eyed Monster: Collins hates the fact that Dickens outshines him as a writer and that he (Collins) is considered a junior protege. When he finally admits that Dickens is a better writer than he is, Collins is even more enraged.
Anachronism Stew: Simmons kills off Inspector Field several years before the real Field died. And he has Collins mention "a militia which the Americans called the National Guard", when American state militias were not organized into the National Guard until 1903.
Arranged Marriage: Wilkie forces the woman with whom he's lived as husband and wife for many years into one of these to get her out of his hair. The guy she marries turns out to be horribly abusive. Wilkie doesn't care, although at the end he helps her kill her husband, then he takes her back.
Body Horror: Wilkie getting an Egyptian scarab inserted into his body.
Call Forward: Collins assumes that the people 120 years or more hence that will be reading his novel will also still be using laudanum, "unless medical science has come up with a common remedy even more efficacious." Medical science did just that when it invented heroin.
Deliberate Values Dissonance: Hoo boy. Wilkie's views on race, sex, and class are all utterly abhorrent by today's standards. Not to mention the usage of old-fashioned terms like "Chinee," "Moslem," and "Hindoo." Dickens is even more racist than Wilkie, but his views on class are fair for their day.
Doppelgänger: Wilkie has one which he calls the Other Wilkie. Or at least he thinks he does. The most bizarre Truth in Television part of the book, as the real Collins actually believed this.
Eagleland: Flavor 2. Wilkie really hates his American reading tour.
Eldritch Abomination: Whatever the spirits are that live in Wilkie's house. One of them eats Wilkie's serving girl. Drood also has elements of this, at least at first.
Face-Heel Turn: Not that he was really a face to begin with, but it's not until Wilkie starts plotting to kill Dickens (and actually does kill his simple-minded serving girl) that he can actually be called a heel.
Wilkie: Tonight I decided to dine relatively lightly and ordered two types of pate, soup, some sweet lobsters, a bottle of dry champagne, a leg of mutton stuffed with oysters and minced onions, two orders of asparagus, some braised beef, a bit of dressed crab, and a side of eggs.
Infant Immortality: Averted in a horrifying scene where Dickens and Collins, looking for Drood in the slums of London, see three dead infant left out by their prostitute mothers to rot.
Mind Screw: What is Drood? What are the spirits in Wilkie's house? What is the other Wilkie? Did anything in the book really happen? Pretty much everything about the book counts.
Opium Den: While exploring the bowels of London (in more than one sense), Dickens and Wilkie stumble into two of these. Wilkie spends a fair amount of time in each later on.
Secret Relationship: Wilkie maintains two: One with a woman he basically treats as his wife, but refuses to marry because he doesn't want the responsibility, the other with a much younger woman who he keeps away from the other.
When fleshing out his new novel Collins thinks about having his detective character be fixated on a post-retirement life of beekeeping.
The parts about Lord Lucan, who was murdered in gory fashion, are fictional, but may be referencing a later and notorious Lord Lucan.
The bits about the play The Frozen Deep about the doomed Franklin Expedition are a shout out to Simmons' previous historical fiction novel, The Terror.
Shout-Out to Shakespeare: Dickens quotes the "Alas, poor Yorick" line from Hamlet. So does Dradles the cryptkeeper, who turns out to be surprisingly literate. Later Dickens quotes the "chimes at midnight" line from Henry IV Part 2.
Through the Eyes of Madness: One likely interpretation is that everything Collins sees and imagines about Drood is the result of his laudanum addiction as well as the effects of Hypno Fool. Agnes' letter to her parents is another clue pointing to this.
Transparent Closet: Wilkie's brother Charley is gay. Wilkie doesn't quite get this, although Dickens (whose daughter married Charley) does.
Unreliable Narrator: Wilkie Collins happens to be addicted to laudanum, and, later, opium. Not to mention that Charles Dickens mesmerizes him a few pages in and never gets around to unmesmerizing him.
Victorian London: Described from the world of fine gentlemen on down to the slums (and below) in gritty, occasionally squicky detail. Special attention is paid to the overflowing sewers and graveyards that are so overcrowded they have to stuff the corpses in on top of each other. The entire book is clearly very well-researched, and many small details of Victorian life are included along with details of both the Collins and Dickens biographies.
Wham Episode: When Wilkie is abducted by Drood, and has the scarab put in his chest.