Lord Darcy by Randall Garrett
Welcome to an alternate world where Richard the Lion-Heart did not die in the year 1199...where magic is a science and science is an art...where the great detective Lord Darcy and the sorcerer Sean O'Lochlainn combine occult skills and brilliant deductions to bring criminals to the King's Justice and thwart those who plot against the Realm. Welcome to a world where murder may be committed by magic most foul, but crime still does not pay - as long as Lord Darcy is on the case.