It is not a classic "spook tale" in the style of M.R.James who was a friend of Ross and to him, by the way, the book is dedicated. The main aspects of the book are an Earl growing mad, an evil Italian woman, a puritan hero, a beautiful and chaste girl, a family curse, a gelatinous creature living in the depths of the marshlands, a ghost of a murdered wife (or what it was :-)) and last but not least an old castle standing in the swamps, besieged by enemies and filled to the roof with dynamite. :-)
Here you have several reviews of the book. On of them likens it to "The Terror" by Dan Simmons or "The Thing" by John Carpenter which I am not sure about but the fact is there is the element of being in a desolate place where on has nowhere to run, with an unknown creature that can strike any time.
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www.goodreads.com]