May 1149. Unexplained death follows Gareth and Gwen wherever they go, as does their reputation for solving murders. So when a man turns up dead at Carlisle Castle, where the pair have traveled as representatives of Gwynedd, King David of Scotland naturally turns to them for answers.

But as the investigation unravels, fractures begin to appear within Gwynedd's proposed alliance, and once again, Gareth and Gwen find themselves at the center of a conspiracy with the throne of England on the line. The Faithless Fool is the fourteenth Gareth & Gwen Medieval Mystery.
Complete Series reading order: The Good Knight, The Uninvited Guest, The Fourth Horseman, The Fallen Princess, The Unlikely Spy, The Lost Brother, The Renegade Merchant, The Unexpected Ally, The Worthy Soldier, The Favored Son, The Viking Prince, The Irish Bride, The Prince's Man, The Faithless Fool. Also The Bard's Daughter (prequel novella).