The Magic Goes Away, by Larry NivenRead about the other entries on the list.
The stories in this collection pivot on a simple but effective idea, straight out of a role playing video game: magic is fueled by a very real and very finite resource, known as mana. As magical spells are cast, mana is consumed—and as mana runs out, magic dwindles. As the energy crises in the real world made headlines, Niven returned to this concept and made the parallels more explicit, but the stories set in this universe all center on the basic problem: managing a limited resource on which the entire world depends for its normal functioning—an idea Piers Anthony, er, borrowed for his Apprentice Adept universe.
--Marshal Zeringue