What is it about the Elric books that makes them so enjoyable and popular?
The Elric books (and indeed, all of Michael Moorcock‘s Eternal Champion fantasy books) have a vigor that is very attractive. Written at a time when the stereotypes of Tolkien and Howard largely dominated the fantasy field, Elric was conceived as a break from those stereotypes: instead of a powerful barbarian who hacks his way to a throne, Elric is weak, a sorcerer, and an Emperor whose actions contribute to the destruction of his nation; instead of pointy-eared elves who appear largely as forces of good, Melnibonéans are a cruel, proud race ignoring their decaying empire and their own decline.
The concept of Law and Order that Moorcock used throughout these books seems to have significantly influenced the roleplaying game D&D in its early stages, and perhaps have inspired later fantasy writers such as Louise Cooper. It continues to inspire roleplaying games today.