The Iron Wyrm Affair. The Bannon & Clare Affairs, #1

Archibald Clare is a detective of truly uncanny abilities-a mentath, capable of feats of deduction and logic that border on the supernatural. He is also abruptly, uniquely, the only unregistered mentath left alive in Londoninium. Someone has murdered the others and, if not for the timely intervention of the Prime Sorceress Emma Bannon, there would have been no one left to stop... whatever is coming.
Mentaths and sorcerers are dying-or worse, being seduced into betraying Queen and Country. Bannon and Clare must uncover treachery, conspiracy, and sorcery of the blackest hue. And in a Britannia where magic has turned the Industrial Revolution on its head, time is short. The game is afoot...