Every Saturday as a child, when Sarah Lippett visited her grandmother’s house, she was transfixed by the objects inside. They loomed before her, seeming to suggest questions that neither they – nor her grandmother – wanted to answer. The window sills were jammed with ornaments. “I’d sit there and work them, like you do when you’re a kid,” Sarah says, waggling her hands as if each again held a porcelain figurine. Paintings crowded the walls and she had the impression that the family knew the artist. But the biggest mystery was a photograph at the top of the stairs. “I can see it now.” There was Nan, with a man. But what was he doing there? He was “really weird and creepy”.

“She was Nan and she lived on her own. I never even thought there was a granddad missing,” Sarah says. She sits on the sofa in her flat in east London, an old photograph album across her lap. “On the other side I had a nanny and grandad, but Nan was …”

Well, she was just Nan, a woman who opened the door with arms full of cuddles and the scent of freshly baked cakes, but who steadfastly refused to talk about herself. Sarah had only one clue that grief lay behind the silence: “I used to catch my mum occasionally crying at Christmas because that’s when she missed her dad the most.”

As the years passed, Sarah, now 31, came to understand that the man in the photograph was Stan, her nan’s late husband, who was never mentioned. But by then the hole in the family tree had created for her a kind of personal uncertainty. She had to find out more.

The result is Stan And Nan, a graphic memoir that charts Sarah’s process of discovery, starting with a sketch of her childhood self – an eager listener in spectacles – visiting her nan at home in Tettenhall, Wolverhampton.

While her nan talks, their mugs of tea enlarge and shrink in the squares of the novel as the emotion intensifies and the details of Stan’s life unfold. Tantalisingly, the bottom edge of two paintings, which Sarah would later find out were Stan’s pictures of Stratford – so detailed “I could sort of place myself in the street” – are just visible on the wall, the frames hovering outside the frame of Sarah’s drawing.

“Did I ever tell you your grandad was the youngest of seven?” Nan asks, and the story begins.

But in reality, of course, it didn’t happen like that.

By 2011, when Sarah became desperate to learn about her late grandfather, she was at art school, too far away to visit. She and her nan had always stayed in touch by letter – “as regularly as I sent them, she’d send them faster” – and so now Sarah began to write. “I thought, I’ll drop it in and see if she responds. I just said, I’d love to know more about Grandad.”

She was unsure how her nan would meet a direct request for information, but before long she had the answer: a letter, written in slanting lines on A4, “in which she told me pretty much the whole story”.

Soon, a second letter arrived, Stan’s life told in vignettes. He enjoyed a bet on the horses with his friend Ernest; he sacrificed a place at art school after an accident curtailed his father’s working life. He courted Sarah’s nan despite her parents’ disapproval of his working-class upbringing. (They met because each day she walked past the fire station where he worked en route to her own office.) “You have probably seen some of his pictures scattered around the house,” Nan wrote. “There are two of Stratford in the dining room ... I love them all.”

The letters contained no single revelation. No great family catastrophe hid between the lines. What really shocked Sarah was how heavily laden the thin sheets of pale blue paper were with love and mourning.

“You know when you read something and you can visualise the way, if someone was telling you, they would look?” Sarah asks. “When someone is talking about someone they love and there’s a spark in their eyes, and telling you about them just brings that person to life? It was really moving and I hadn’t had that from my nan ever. I think she lost that spark when he died. That’s what the letters brought out for me. This love carried through.”

Sarah wrote again, asking for more details. This time a letter came back with the briefest of answers. “And then she wrote – and obviously it was too much for her – because she wrote, ‘This is the last letter I’m going to send to you about your granddad.’ And she underlined it. Last.”

Sarah began to draw, and on her next visit to Nan, now 92 and ill at home, she took sketches, including one of Stan. Nan laughed. But the drawing was difficult. “Nothing was working,” Sarah says.

She had found out about Stan, but a new mystery bloomed in its place. Now Sarah wondered why she knew so little of Nan’s life?

“When I got older, I wondered why didn’t Nan remarry? Why didn’t she meet anyone else? Why didn’t she go out and do anything?” On visits, Nan always quizzed Sarah about life – “Tell me what you’re doing! Tell me everything!” – but revealed nothing of her own. “My nan left me this in her will,” Sarah says, bobbing out of the room. She returns with an ornament, a small porcelain girl: “Nan had it because she thought it looked like me when I was little. I always had my thumb in my mouth and a little blanket I carried around.” She places the ornament beside her on the sofa.

In the novel, Sarah transforms silence into strength. Events of magnitude are conveyed with brutal, wordless economy so that Nan’s demise comes quickly: falling, fallen within two squares. Sarah’s eyes are shiny as we turn the pages. Nan died in 2013, shortly before her 95th birthday.

It is heartbreaking for Sarah that she didn’t live long enough to see all the drawings of Stan, to read the finished story. But then the book would be a very different thing because it was at the funeral that Sarah really began to understand her grandmother. “I met people who could help me piece together all the bits that she had left empty for me.” She even met Piggy, one of Nan’s old schoolfriends. New letters hit Sarah’s mat, the envelopes addressed in a hand heartbreakingly similar to Nan’s.

“I think I accepted them more, the decisions that she made with her life. Why she didn’t want to talk. It was just too painful.” Stan was only 52 when he returned early from work, feeling off colour. Within a few hours he lay dead on the bed. “She lost so many years with him,” Sarah says, laying a hand on a sketch she made from a photograph of her grandparents together. He is knee-deep in the sea, giving Nan a piggyback. “I don’t think I’ve ever seen her laugh that much,” Sarah says. “She’s just totally … loose, lost in that moment.”

One of the book’s most moving sequences shows Sarah at her nan’s hospital bedside. Stripped of colour and words, square after square shows her clutching, grasping, covering, stroking her nan’s hand. “I was talking to her but I don’t know if she could hear me. So feeling her hand and feeling the warmth of her hand and having that contact …” she trails off. “I held her hand, and just talked to her in my head.” There are so many pictures of these last moments of touch, as if the pencil itself cannot bear to let go.

Sarah says she loved being able to make things happen for her nan in the book that didn’t happen in life. Stan’s old hankering to open a cafe, where he could sell his paintings while Nan baked her cakes, is rendered here in a way that gives life to the dream. When Sarah showed elements of this work in her graduation show at the Royal College of Art, she felt she had got Stan into art school at last.

A life has been coloured in. Even Stan’s “pictures of Stratford” are blank canvases no more. By the end of the book, Sarah has sketched inside the frames. Maybe, she says, it’s time to hang one on her own wall.

Stan and Nan by Sarah Lippett is published by Vintage, £16.99). To order a copy for £13.93, go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £15, online orders only