Looking at the title you might mistakenly think this book is about Idun, Freya's and Thor's daughter. It isn't. Idun never gets a chance to grow into a whole character, she's reduced into a plot point in her parents tale as are other characters too.
That shouldn't be a problem, because I loved Freya, but years have changed her into something unrecognisable. I guess the power and might of the Moon goddess, and personal sacrifices can do that to a woman, but this tragedy too is downplayed into lunacy.
The story ends up being about no one and everyone. Most of all Idun is about Scandinavian bronze age and the legends they left behind. The world is disturbingly real, but it's not enough. This reworking of old Nordic myths needs more character.