The Magic Goes Away – Larry Niven

In ancient times, there was magic in the world. But the supply of mana, on which magic is based, is dwindling. Creatures with magical metabolisms, such as dragons, are in serious trouble, and in general the world is becoming a less mystical place. A group of adventurers sets out to find the last remaining source of mana.

The idea underlying the novel is very clever. Unfortunately it is not very good. It is based on the short story Not Long Before the End but the idea doesn't scale very well to a full length novel.