In 2000, Guillaume Simoneau, a French-Canadian photographer, met an American girl called Caroline Annandale at a Maine Media Photography Workshop. They fell in love – but their "feverish" relationship took a strange twist when Caroline enlisted in the US Army just after September 11, and was shipped to Iraq. Left behind, Simoneau nursed feelings of heartbreak, abandonment and deep anxiety about her safety. Then, things started to fall apart, not least because of what Caroline experienced as a soldier. Later, they reconnected – and a long-distance, but no less turbulent, relationship ensued through emails, letters, text messages and photos.

When asked recently what he wanted his photographs to say to people, Simoneau replied: "That everything will be OK." One senses that this message of hope and acceptance came out of the making of Love and War, Simoneau's photo book that wears its heart on its sleeve. It takes the viewer on a journey into a passionate, turbulent relationship – where it becomes increasingly clear that everything is not going to be OK.

Mapping heavy-duty emotional territory is a tall order for any photography book, even one with words scattered throughout that act like clues. But Love and War works, perhaps because it makes the viewer strive to piece together the lovers' narrative.

Love and War is as far from a book of war photography as it is possible to get, but it is very much a book about war. It begins with a head and shoulders portrait of Caroline in 2008, in which she stares not so much at the camera, as right through it. She has survived, but something has been lost along the way. As the book's narrative unfolds in its wilfully non-chronological way, we see another Caroline, younger and carefree. In 2000, she is captured crouching in a grassy meadow, a flower in her mouth. The same year, she coquettishly pouts for Simoneau's camera in one frame and is caught daydreaming in the bath in an intimately relaxed black-and-white nude portrait. The camera (and Simoneau) adores her, and she seems relaxed in its gaze. There's a tenderness to these images that makes them arresting when they appear haphazardly throughout the book, like fragments of a love lost. Which, of course, is what they are.

Caroline's world by Joanna R, Rockport, Maine, 2000 Photograph: Guillaume Simoneau

The post-Iraq portraits of Caroline show a woman who has cast off her girlishness, but also seems more defiant before Simoneau's camera. In one, she looks sombre in shades and long grey coat on Veteran's Day; in another, she poses with a handgun. How much this change has to do with her experiences in Iraq or with the unravelling of their romance is hard to say.

The narrative is punctuated with other clues about her uncertain state of mind. A text message from Caroline to Simoneau in the summer of 2008 reads: "The more I think of where we are heading individually, the more I believe in us being together." Another, from a few months later, reads: "I seriously can't think of you for very long. I honestly feel wrong physically." This is the universal language of decaying romance, of course, but then you begin to wonder if the latter text may be hinting at other, darker traumas.

Symbolic landscapes, too, dot the narrative: a magnolia tree on a street corner in Montreal; a fractured globe standing beside an orchid on a chest of drawers, which Simoneau has titled Broken Vows. That image was made in 2009 in Lévis, Québec. It is the book's penultimate image, followed by a portrait of Caroline standing to attention in full fatigues. She looks uncomfortable as she stares out of the frame. It is painfully and ironically titled, Wearing Army Uniform for Me, Kennesaw, Georgia, 2008.

If there is a moment of high drama in the book, it comes not from a photograph but text: an email written to Simoneau by Caroline's mother in which, in an almost casual way, she tells him that: "On May 20, 2003, [Caroline] married her friend, Joe Hopkins, changing her name to Caroline Ralston Hopkins ..." That the two reconnected at all after this news says much about Simoneau's dedication to her – and his photographic project about her.

The more I think text, summer, 2008. Photograph: Guillaume Simoneau

Love and War is a book loaded with subtexts. And it leaves so many unanswered questions hanging: did Caroline change in the eight years because of her experience of war or because she simply matured? And given that they met on a photography workshop, why is there only one image by her here – portrait of Simoneau covered in a cloth behind his plate camera making one of the images for the book.

The book's final two pages may provide the answer, suggesting that writing, rather than photography, may be her gift. Here, she records her recollections of comradeship and death in Iraq, sparingly and with great force. "As a young girl, I always dreamed of a white Christmas. As a soldier standing in ceremonial formations that Christmas Eve, it crossed my mind that my fallen comrades might arrive home for a snowy holiday burial. I was scheduled to be stuck in this hellhole for 10 more months. But at least I was alive."

For all that, Love and War is a story told principally through Simoneau's eyes, his camera – and the prism of his heartbreak. But that is its great strength. It is a narrative about love and war told by someone who experienced one but not the other – and who was left behind by both. It is a story about what war does to love.

Love and War is at VU Photo, Québec, from 6 September to 6 October.

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