DotD is one of the better books of the Ravenloft series, the best at the time it came out (though subsequent releases Heart of Midnight and I, Strahd: the Memoirs of a Vampire easily topped it, in my opinion) and is Christie Golden's best novel in the series.
Why?
Well, for starters, it strays from the beaten path (Barovia and Vampire of the Mists) and doesn't toss previously-written history out the window (Nova Vaasa and Sir Tristen Hiregaard/Malken and The Enemy Within). It takes us instead on a magical riverboat (a novel idea, pardon the pun, for a book) into the mysterious island domain of Souragne, and does a fine job integrating the French/Louisianan culture into the domain, whose roots seem to stem from there.
She also gives us wonderful characters: Larissa, Willen, the Maiden of the Swamp, and even Anton Misroi. As well, we receive chilling villains: Captain Dumont and Lond. The swamp domain is well-fleshed out, and even the diffences between Misroi's previously-published past and the one presented here are forgettable (Domains of Dread, the current sourcebook for RL, takes this story over the others, for good reason).
In all, DotD is a fantastic book, with unique ideas and a well-developed, well-told tale. My only complaint is that it wasn't as long as VotM.