According to the Dictionary of Mythology by J. A. Coleman (Arcturus Publishing, 2007), a ghoul is:
a demon preying on the dead: a fiend.
But that doesn't tell us a whole lot about ghouls. What are they, really? What do they look like?
Sources seem to be divided on this. Half state that a ghoul is a monster similar to a zombie (as they are seen as secondary characters in Kresley Cole's Immortals After Dark series). They are mindless monsters intent on infecting others with their highly contagious disease.
Clearly not the subject of many paranormal romances.
The second source states them as defilers of the dead. In other words they prey on the dead to eat (and because the you-are-what-you-eat analogy, they become the Undead). Some sources state they are solely female, others that they can be of either gender. Some sources state that they are able to transform into the guise of animals.
-The Wikipedia page
-The Vampire by Montague Summers, pages 231-237.
Unfortunately, I cannot name any paranormal romances featuring ghouls as the protagonist(s). Does anyone out there have one to recommend?