Author and journalist Leah McLaren was a precocious 13-year-old when she broke down at her mother’s kitchen table one night in Toronto and described a harrowing sexual experience at a pool party. Her mother, Cessie, brewed her a mug of herbal tea, added a slug of whisky, and countered it with a tale of her own. At just 12, Cessie had been raped by her riding instructor. The Horseman, she called him. Having groomed her for assault, he then persuaded her that she was in love with him, continuing his abuse until she was 15.

Listening, McLaren’s first response was relief – perhaps her own story wasn’t so momentous after all. Her mother’s, on the other hand, would come to haunt her. It appeared to hold the key to Cessie’s doomed marriage to McLaren’s small-town father at 21 and her flight, a dozen years later, in pursuit of urban sophistication and a career in newspapers. It explained a tortured romance that saw Cessie lurch from one emotional crisis to another throughout McLaren’s adolescence. It even accounted for a parenting style that, while superficially resembling the kind of mother-daughter bond lauded in pop culture by the likes of Gilmore Girls, was so free of boundaries it left McLaren unable to separate her mother’s feelings and problems from her own.

“The Horseman was both the clue and the final reveal. He was keystone in the arch, the signature at the bottom of every page. As Homer Simpson once observed of beer, the Horseman was the cause of and solution to all of life’s problems,” explains McLaren. It could only be a matter of time before someone in this family of writers (the author’s mother and uncle are both journalists and writers) published a book about it.

Where You End and I Begin isn’t in fact that book. Instead, it’s something more amorphous, more exposing. It started as a collaboration between the author and her mother but after Cessie withdrew, it ceased being a journalistic investigation into the Horseman and his crimes (there were other child victims) and became an intimate voyage into the deepest, darkest heart of motherhood and daughterhood, musing too on consent, victim narratives and the ownership of stories. The result is a work of probing insight and undaunted compassion; one that’s fearlessly engrossing, frequently funny and sometimes plain hair-raising.

An account of McLaren’s efforts to win her mother’s blessing for the book over a girls’ weekend in New York frames a narrative composed of chronologically arranged vignettes that capture telling moments from McLaren’s girlhood and early adulthood. More room mate than parent, Cessie gave her the freedom to cruise through school snacking on magic mushrooms, stay out all night without calling home, and tolerate the gaze of a voyeur in the alleyway outside their apartment – until she woke to find him trying to climb through her bedroom window.

Gross-out humour meets Jungian psychology as the book moves between a succession of vivid backdrops, among them Vogrie, a dilapidated farmhouse that her mother looks after for a spell. Stuffed with treasures such as beaded flapper dresses and original Chagall sketches, it’s “a glittering, self-contained universe, a cobwebbed palace of infinite internal escapes”.

When it comes to her mother, McLaren’s gaze is acutely honed. These days, she is “a midsize sedan of a woman”, but in younger years she was “a slim, freckle-tanned flyaway-blonde with devouring blue eyes” who captivated her daughter with her wit and spirit. We’re “poor with panache”, she would insist, taping words she’d chosen for a family motto to the fridge door: COMMITMENT SUCKS THE LIFE RIGHT OUT OF YOU.

And yet Cessie craved male devotion, and as McLaren grew up she found herself cast in an unsought role, adding rival to a list that already numbered confidante, best friend and honorary sister. “The only thing I have ever really wanted is to be recognised by her as what I am and always will be: her daughter,” she confides.

Becoming a parent herself changed things, and though it hardened McLaren’s resolve to act differently, it also allowed her to empathise with some of Cessie’s maternal ambivalence. By the end of this white-knuckle ride of a book, the author is finally able to disentangle her own youthful misadventures from her mother’s trauma.

This memoir does demand a coda, though. How will it affect McLaren’s relationship with the woman who bore her? In spite of everything, it’s hard not to find yourself rooting for both parties in this untamed, yearning story of imperfect mother-daughter love.

• Where You End and I Begin by Leah McLaren is published by John Murray Press (£16.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply