Mendaga's Morning (Sphere; 1979) is not the "super-shock" novel the marketing tells us it is, but it is a suspenseful yarn of possession with an emphasis on the psychology of possession and the possessor.
A little research reveals that Mr. Farren is a former Jesuit who has written extensively about witchcraft and the occult.
Mendaga's Morning, which sports a bloody good cover and is a speedy read at 176 pages, is a story about a little girl possessed, ala The Exorcist and a stack of similarly themed novels, but the source of its villain/demon is one conjured by a specific coven, not God, or the Devil.
With its emphasis more on the psychological impact of a possession, the book eschews the grotesque and hyper-gory for a quieter, more penetrating trip down Witchcraft Lane.
It shares an interesting parallel with Michael McDowell's first book, The Amulet, in that a connecting physical element in the story is an old amulet. The connection is probably coincidental as McDowell's book was published in the US in 1980 and Farren's was published in the UK a year earlier. On his blog, Farren mentions that he wrote the book in a couple of weeks, a case of the characters taking the reins for him, so to speak.
The book is well worth a read, and has not been re-published since its debut.