When Skadi came to Asgard for vengeance or compensation the Æsir agreed to give her a husband from their ranks. But she had to choose solely by his feet. This is why Harris mentions Njord's shapely feet.
Loki and Odin were traveling in Midgard one day when, arguably because of his chaotic blood, Loki threw a stone a fishing otter and killed it. The otter turned out to be a shape-shifting human and his family demanded compensation — enough gold to cover every hair on an otter's pelt. This is the "otter's ransom" mentioned in the book.
Thiazi (Skadi's Giant father), longing for the fruit of the Gods, made Loki give him access to Idun and her apples. He came in eagle guise and kidnapped Idun, taking her to Jotunheim, home of the Giants/Ice People. To save her Loki had to borrow Freyja's feather cloak, turn into a hawk, turn Idun into a hazelnut, and carry her back to Asgard. This is what Frey refers to when he suggest shape-shifting Idun for faster transport under Red Horse Hill.