In Episode 12, Albert turns on a lamp in front of an old music flier. The flier is written in Serbian, and advertises an actual venue located in present day Belgrade, called Kolarčeva Zaduzbina.
The opera seen in Episode 1 is Gaetano Donizetti's Lucia di Lammermoor, with the famous Mad Scene being highlighted.
The Count's Emerald pill-case is lifted directly from the novel, in which the Count has a hallowed-out emerald in which he keeps hashish. Also various other details--the samurai armor in Albert's room, is also mentioned in the novel (well, some sort of armor of oriental origin, which could have meant it was from almost anywhere in the general vicinity Asia given the era the novel was written). Various other visual details and even a few snippets of conversation make the transition exactly, which is actually kind of surprising considering the change in focus and setting the series underwent, and besides which shows the attention to detail of the crew involved. On that note, should we add a "shown their work" entry to the main page or something?
On that note, if you pay very close attention, a few characters from the original book pop up - even if they don't actually appear in the Anime. One of the sailors listed in the original Morrel crew is "Jacopo" (identifiable only by his signature on a picture, which is a bit obscured but definitely there), who was the Count's first 'helper' in the novel. Gankutsuou seems to take on the role that Abbe Faria had in the novel, and even appears with a cross around his neck - although that comparison is arguable, considering what Gankutsuou is. Abbe Busoni makes a appearance as well, although seemingly as Luigi Vampa's alias, rather than one of the Count's many alter-egos.