When Karen Campbell left Glasgow University in 1987, she did what any left-leaning, vegetarian liberal (her description) with a degree in English and French would do. She joined the police force.

“It seemed like a job where no two days would be the same,” she tells me over the phone from her home in Dumfries and Galloway. “For someone that wasn’t sure what they wanted to do it would be interesting and exciting. So with the gung-ho attitude of youth, I just thought: why not? I graduated one Tuesday and joined Strathclyde Police the next Tuesday, and then spent five and a half years thinking: ‘I’m not sure this is the right choice’.”

In fact it wasn’t that strange a choice. Nor, as it turned out, was it a wrong one. For a start, both her parents had served in the Glasgow force so it was a job she knew. And she had another reason for joining beyond familiarity and that “gung-ho” confidence.

“People sometimes laugh when I say it, but I was genuinely torn between social work and the police,” she says. “Folk think ‘That’s mad, how could the police be comparable?’ But for me it was – that sense of doing some good, making a difference in society.

“With my dad I knew the sorts of things that cops did. I knew that it did make a difference. Literally, the buck stops with you, as a cop. That never left me and it’s something I’m really proud to have done. But I wasn’t very street-smart. I was book-smart, but I wasn’t a good thief-catcher, put it that way.”

Campbell quit the force in the 1992 and didn’t return. It was the usual story. She had two young children under two and was facing an employment structure which hadn’t yet come to grips with ideas like return-to-work pathways and part-time working.

But law enforcement’s loss would turn out to be fiction’s gain. Campbell returned to a teenage habit – writing short stories – and later enrolled on Glasgow University’s fecund Creative Writing post-graduate course. There she encountered a sort of literary Dream Team in the form of then-course leaders Alasdair Gray, James Kelman and Willy Maley, and enjoyed frequent visits from writers of the calibre of Janice Galloway and Liz Lochhead.

The end result? A run of four novels in three years featuring young police woman Anna Cameron. A newly-promoted sergeant in the Glasgow force when we first meet her in 2008’s The Twilight Time, she’s a Chief Inspector by 2011’s Proof Of Life, the novel which closes the quartet. Over the course of the four books she deals with a messy private life and investigates cases involving sex workers, racism, terror threats and, of course, murders.

Despite that – and despite being shortlisted for prestigious annual crime writing award the Gold Dagger – Campbell doesn’t view herself as a crime writer and never has. And she certainly doesn’t think of the Anna Cameron books as crime novels.

“They were very much about social issues, they were about the world I saw as a cop,” she explains. “I wanted to write about what it was like to be a police office and my experience as a young female. So I didn’t come to it with any preconceptions at all. There’s quite a lot of slow bits in those books. Obviously you’ve got to write fiction, you’ve got to have a narrative. But I wanted them to be quite reflective.” There’s the frustrated social worker in her again.

Campbell followed the Cameron quartet with three more novels, including one set in Italy during the Second World War. Now comes Paper Cup, a new novel with a new publisher – Edinburgh-based powerhouse Canongate.

The story centres on Kelly, a woman with alcohol dependency issues who is living rough in Glasgow when the novel opens. Huddled on a bench in George Square, she encounters a young woman on her hen night, listens silently to the woman’s drunken ramblings and then, after she has left with her friends, finds the diamond engagement ring she has dropped. She tries to post it – difficult when the teller doesn’t like the look of you and the only address you have is ‘Susan, who’s getting married, Gatehouse of Fleet’. Then she decides to sell it instead, but that doesn’t go well, either.

One thing leads to another and soon Kelly finds herself leaving Glasgow and heading for Portpatrick in Dumfries and Galloway – journeying towards Gatehouse of Fleet and back into her own troubled adolescence in Kirkcudbright. She walks, she hitches, she thinks. She picks up a four-legged companion and she undertakes a pilgrimage of sorts, circling slowly towards – well, that would be telling.

“I wanted to write something about vulnerability,” Campbell explains when I ask her about the starting point for Paper Cup. “A kind of kernel of an idea can just be an emotion. Then you circle round it and you start thinking what would be the best way to explore that? I thought about somebody who is just stripped of everything, exposed.”

As for Kelly, she just “roared onto the page”, as Campbell puts it. “I don’t know where she came from but I was very grateful when she appeared. She had a real spirit about her and that was something I was very keen to write about – that somebody might be down and they might be broken but it doesn’t mean they are beaten.”

What compels Kelly to pick herself up and act is in part an event she witnesses, a terrible accident in which a double decker bus mounts a pavement near George Square and ploughs into pedestrians. “Glasgow is in shock,” Campbell writes. “You can feel it permeate the city’s walls, turning glittering mica into tears.”

You don’t have to think hard to see the real life inspiration for the event – the death of six Christmas shoppers on December 22, 2014, killed when a bin lorry crashed into them on Queen Street. A further 15 people were injured. A shocked city did cry.

“I was in two minds about how close I would make it and that’s why I changed bits of it,” she says. “But I think for me it was [about] the aftermath of that accident – the sense that the whole city of Glasgow was grieving.”

As for the novel’s title, it came from a project Campbell undertook for Scottish PEN, the international body championing literature and freedom of speech. Campbell was one of a team of writers running writing workshops with marginalised groups. Already well through the novel at this point, she was paired with former rough sleepers and was struck by the power of an image in a poem written by one of them.

“It was pretty much about having your life in a paper cup. It stayed with me. The cup is fragile but resilient – you can put hot water in it, it can endure, but it can just blow away really quickly. It was such a moving piece that this person had written. I don’t think it influenced the book because I had written most of it. But in terms of underpinning what I had wanted to write about and what I felt about what I was writing, it reinforced that. It made me think everybody’s story matters.”

It’s a lesson worth remembering – and one which Paper Cup’s plucky, knocked-about heroine ably demonstrates.

Paper Cup is out now (Canongate, £14.99)