This is a remarkably good book, that I somehow failed to enjoy as much as I wanted or expected, but I think the failing is mine, rather than Byatt's.
"The thin child in wartime" (Byatt herself) is given a book of Norse legends, that she treasures. Those stories are retold through her eyes and thoughts, interspersed with snippets about her own life, told in a similar epic, mythical, Silmarillionish style, weaving occasional lines of liturgy and hymns into the prose (as myths weave into each other and ourselves). It dips in and out of myth, but somehow I didn't feel a narrative pull.
She is a thoughtful child, with a vivid imagination and an analytical questioning mind, comparing the gods of legend with the Christian one she learns about at school and church. "In the story told in the stone church a grandfatherly figure who resented presumption had spend six delectable days making things." She notices that characters come in threes, that there are two ways to win battles ("to be surprisingly strong, or to be a gallant forlorn hope"), and rules in stories exist to be broken. She treats all myths, including Christianity, like fairy stories, "these offered the pleasure to the mind that the unreal offers when it is briefly more real than the visible world can ever be." The only thing alive in the church is the English language.
The war brings intellectual conflict, as well as more visceral fears, especially for her fighting father: "She asked herself who were the good and wise Germans who had written 'Asgard and the Gods'" and wondered how she could trust "the storytelling voice that gripped her imagination, and tactfully suggest explanations."
If young Byatt really thought as the thin child does, it's no wonder she became a storyteller. "Part of the delight and mystery of this book was that everything was told several times, in different orders and in different tones of voice... It is told in the present tense, a prophetic vision of the future, seen as though it was Now. The think child became an onlooker in the death of the world... It felt different from Christian accounts of the end of things... Here the gods themselves were judged and found wanting." And to show her erudition as well as her empathy, there is an essay about mythology at the back of the book.
She has fun with the gods' quirks, especially Loki's mischievousness: "Chaos pleased him... He would provoke turbulence to please himself and tried to understand it in order to make more of it. He was in burning columns of smoke in battlefields. He was in the fury of rivers bursting their banks, or the waterwalls of high tides throwing themselves over flood defences, bringing down ships and houses."
The language is is rich, vivid and beautiful, especially when describing plants, animals and water ("The flung snake fell through the firmament in shifting shapes... her mane of fresh-fronds streaming back from her sharp skull, her fangs glinting."), but I expect that from Byatt. It is bound, printed and laid out with a strong eye for aesthetics. There are a few lovely pen and ink drawings to add to the images she conjures in the reader's mind. The parallels between the thin child's life and what she reads are clear (Ragnarok is the end of the world, and WW2 seemed as if it would be too), but mostly subtle. I can't fault it at any level, other than that it disappointed me, or perhaps that my reaction disappointed me. Perhaps I shouldn't try: "This is how myths work. They are things, creatures, stories, inhabiting the mind. They cannot be explained and do not explain; they are neither creeds nor allegories."