A short collection of Lord Peter Wimsey-esque murder mysteries — four stories, written and set between 1964 and 1973, with two gimmicks: Richard the Lionheart didn't die in 1199, and a small (but significant) fraction of the population are able to use magic. The former means that we're sitting in the middle of an Anglo-French feudal culture, and the latter means that technology has developed in a rather different direction.
This doesn't mean that we get a bunch of mystery stories revolving around the use of magic, though; instead, magical forensics is a well-developed science, and magic-based engineering is available for those who can afford it. My favourite example was the investigator's electric torch — which is normal in every regard except that the filament is prevented from burning up by an enchantment rather than a vacuum…
The only complaint that I've got are that these aren't fair mysteries (that is, there are clues that the investigator and his assistants know but the reader doesn't). I'd like to see the same idea done at novel length.
ISBN 0441545416.