The Australian writer Hannah Kent has found critical and commercial success with fictionalised reworkings of real-life historical crimes. Her bestselling debut, Burial Rites, shortlisted for the 2014 Women’s prize, examined the case of Agnes Magnúsdóttir, condemned to death in Iceland in 1829 for the savage murder of her master. Her second, The Good People, was based on 1820s newspaper reports about the violent attempts by an Irish village to banish a child they believed to be a changeling. Both books cleaved closely to the historical record, working within the constraints of the known facts to invest those bleak and brutal stories with ambiguity and depth, and to give a voice to participants whom the past had long condemned to silence.

With Devotion, Kent returns for a third time to the same period, this time to the Prussian village of Kay and a close-knit community of Old Lutherans, compelled by the strict religious reforms of their emperor to worship in secret. Many of Kent’s familiar themes are here: the fierce connections and exclusions that bind small communities; the tension between doctrinal religion and superstition; the power of landscape. This time, however, the history is much closer to home. Kent grew up (and still lives) in the Adelaide Hills in South Australia. Most of the émigrés who settled in this unceded Indigenous land were English, but some were Prussians like the Old Lutherans of Kay, in search of a place where they could practise their faith in peace. Those Prussians were Kent’s forebears – the start of her own story.

This novel is different in other ways, too. Kent originally conceived her third book as an exploration of the power of female friendship but, in 2017, as Australia voted in favour of same-sex marriage and her girlfriend proposed, she realised that she wanted to go further, to celebrate the love between two women with “music instead of silence, presence instead of absence”. Devotion is, from its opening pages, a passionate celebration of that love.

Fifteen-year-old Hanne has never fitted in with the other girls in the village: they “came together in a dance she did not know the steps to”. A wild soul with a connection to nature so deep that she hears the songs of the trees and the bell-chime of snow falling, Hanne has come to believe that she is “a cuckoo born to a songbird”, someone who cannot properly be loved even by her own mother. Then she meets Thea.

Kent describes Devotion as “a return to the light after two books that have very much considered the dark”, but, from the outset, there are shadows. Before Hanne and Thea even meet, long before they embark with the rest of the village on the gruelling journey to Australia, their story is an elegy, freighted with loss and longing, the smallest of shared moments heavy with significance. At their best these moments are piercingly beautiful. When Hanne confesses that “Thea was as a chink of light in a curtain. When I put my eye to her, the world beyond blazed”, her words cut cleanly to the heart.

There is poetry, too, in Hanne’s profound engagement with the natural world. She writes of “greasy-fingered light” on the mudflats, the wind wanting “to drag her into a dance”. Like Hanne’s love for God, her love for Thea is bound up in landscape, a part of both its glorious vastness and its tiny, perfect details.

It is a pity, then, that too often Kent allows the twin raptures of faith and physical love to get the better of her prose, whipping it into ecstasies that come dangerously close to pastiche. “With the firm grip of [Thea’s] fingers between my own,” Hanne gasps as they set sail for Australia, “I felt time dissolve in the arms of the ocean’s brilliant, salted constancy.”

The larger difficulty with Devotion, however, is that, despite the vast distances travelled by the people of Kay, the narrative never quite takes flight. Kent keeps some surprises up her sleeve, upending our expectations of where this story will go, but her large cast of villagers remain fixed in their roles. Their odyssey is gruelling – Kent has done her research and does not spare the detail – but no one really changes. Those who started out good stay good and in the same ways. The bad continue to be bad.

Hanne is transported by her love for Thea and their relationship opens her up to become fully herself, but that self is essentially unaltered. Hanne’s passion is absolute, unwavering, untainted by the doubt or fear essential for self-revelation. The reader is left craving more of the darkness of Kent’s previous work, darkness that might have defined Hanne more clearly and ensured that this sweet misfit seized our hearts and imaginations as Agnes, the violent murderer in Burial Rites, so powerfully contrived to do.

• Devotion by Hannah Kent is published by Pan Macmillan (£14.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.