I don’t know about a modern book, but a similar story appears in either the Prose Edda or the Volsunga Saga. Possibly both. The gods are out hunting when they kill an otter, only to find that it’s actually a man who has changed his shape into an otter. The gods then offer to pay compensation to the otter’s father, who demands the skin of the otter be filled with gold, and then covered up with gold. Loki then robs a dwarf of his treasure to get the money for the payment, and the dwarf curses the wealth, which is the origin of the curses Nibelungen horde, which I believe includes the titular ring from Wagner’s ring cycle. The otter’s brother then kills his father to steal the horde, and turns into a dragon to protect it. He is called Fafnir.