Too Many Magicians,

Set in an alternate reality where Richard the Lionheart's descendants rule the Anglo-French Empire, the laws of magic have developed in place of the laws of physics. In this late 20th century world, people still travel through pea fog by horse and carriage, but magic has made levitation and enchantment spells the norm, especially at a sorcerer's convention. The International Sorcerer's Convention is in full swing. Until London's Chief Forensics Sorcerer, Sir James Zwinge, is found dead inside a locked room, that is. Master Sean O Lochlainn had been at odds with Sir James over competing breakthroughs in incision-free surgery, making him the prime suspect. It's up to Lord Darcy to solve Sir James' murder and exonerate his own forensic sorcerer and trusted assistant. But the mystery deepens when the murder of a Naval spy is connected to that of Sir James' death. Conflict with Poland, a dominating world power, changes the game and suddenly there is a lot more at stake than the freedom of Lord Darcy's old friend. Too Many Magicians was nominated for the Hugo Award in 1967, and the complete Lord Darcy series later won the Sideways Award for Alternate History.