The Wolf’s Lover by Samantha MacLeod

Karen McDonald buried her past.

Five years after her divorce, Karen is a wildlife biologist studying wolves in Yellowstone National Park. But when the wolf she tranquilizes turns into a man – a man she recognizes from the recurring dreams which have haunted her for years – her rational, scientific world gives way to something stranger and more dangerous.
The man is Vali, son of the Norse god Loki. He was imprisoned thousands of years ago, cursed to live inside a wolf’s body. Now Vali tells Karen he’s being hunted…and he’s hunting as well.

Because something has awoken beneath the volcanic caldera of Yellowstone, an ancient power that could spell the end of everything, unless Karen is willing to risk her life, and unearth the secrets of her past, to save her world.