The Excalibur Murders — J.M.C. Blair

Throughout Camelot, he is whispered to be a magician, a sorcerer, a wizard. But Merlin is merely a scholar, a practitioner of medicine, and King Arthur's trusted advisor. Though he possesses no otherworldly abilities, his keen intellect grants him certain powers of deduction...

Though he labored to help Arthur unite all of the Britons under his rule, Merlin's work is not done. The vassal kings are a contentious and superstitious lot who have not easily accepted Arthur as their liege. And Arthur himself, to Merlin's distress, is still prey to the old beliefs. The king is convinced that he can bring peace and prosperity to Camelot with the Stone of Bran, an ancient skull-shaped artifact purported to be magical.

But when it is stolen from the king's own private chambers-- along with the legendary sword Excalibur--and one of Arthur's squires is brutally murdered during the theft, Merlin, and his apprentice Colin (the maiden Nimue in disguise) must use the power of reason to conjure up a miracle and catch a murderer...