I would strongly advise that this trilogy be read in order - Incubus, Water Witch then Dark Possession, as the author develops the characters and relationships throughout the three books.
After Water Witch I started on the Dark Swan Rising (written by Carol Goodman and her husband hence the author name Lee Carroll) trilogy and was very disappointed in the way it ended - I was left with more questions than were answered and felt the last few chapters were rushed. Due to this, and my love of the first two Fairwick Chronicles, I was very wary of reading Dark Possession however the book is fantastically written, answers all the readers questions (and more) is, in parts, very emotional and was, simply, a perfect end to the trilogy.
I don't want to spoil the trilogy for others so all I will say is it is a beautiful blend of fairies, demons and humans with romance, conflict, betrayal and compassion. It is a page turner and, having read it 3 times now, one that you will never tire of. HIGHLY recommended