The Narrative of Arthur Gordon Pym of Nantucket by Edgar Allan Poe
According to the back cover, this is Poe's only novel. I can see why.
Considering the novel is only around 180 pages, it contains several continuity errors (one character gives the narrator a piece of information, despite the fact that the said character was killed off in a previous chapter). The story is also padded out with long sections of irrelevant detail, such as how to correctly stow cargo in the hold of a ship to prevent it shifting during storms, and how penguins and albatrosses build their nests, all of which interrupt the flow of the story for no apparent reason. These technical sections seem to be lifted wholesale from the works of other writers.
The ending is also sudden and ambiguous, with the final 'Note' chapter explaining that the last few chapters of the story are missing. It seems to me that Poe was not comfortable writing a novel-length story and was in a hurry to finish as soon as possible, thus explaining the lack of care given to continuity, the padding, and unsatisfactory ending.
The front cover blurb of the edition I have read states that Jorge Luis Borges thinks this is "Poe's greatest work.” All I can say is, he must have read a different book to me. I think Poe made the right decision to stick to short fiction and poetry.