There have been three people in Gavin Bell's marriage lately. Gavin, his wife Laura, and a third party named Carter Blake. Actually, make that four: throw in Mason Cross as well.

Bell has been busy completing his third Carter Blake novel and sending it off to his publisher. "I was so focused on it for ages, excluding all else," he laughs. "Laura was absolutely sick of it after months of me working on that and nothing else in my spare time."

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And Mason Cross? That happens to be Bell's nom de plume.

This week, he launches the second Blake thriller. The Samaritan, it's called. It's set in Los Angeles; Blake is a manhunter, a shadowy figure with all sorts of useful skills, who volunteers his services to the LAPD as they pursue a serial killer, an equally shadowy figure from Blake's own past.

The new book is launched at Waterstones Argyle Street two nights from now. Bell has high hopes for it. The first one, The Killing Season, sold well and brought him to the attention of Lee Child, bestselling author of the Jack Reacher novels. "My kind of book", was Child's verdict.

It's all happening for Glasgow-born Bell just now. He'll be guesting at the Edinburgh International Book Festival on August 21 and at the Bloody Scotland festival, in Stirling, on September 13.

Now 36, he has spent the last decade in the voluntary sector. He currently works in IT for the Scottish Council for Voluntary Organisations at its offices in the Merchant City. Home is in Cambuslang, where he lives with Laura and their three children.

He was good at creative writing in school but later abandoned that enthusiasm, only picking it up again in his late teens when he was studying English at Stirling University. A key catalyst was Stephen King's widely-admired book On Writing.

"It's an amazing book about King's own journey towards becoming a writer but it also has useful advice for writers," says Bell. "It inspired me to give writing another go and start writing more seriously."

He submitted short stories to magazines and competitions and although he received "a few glowing rejections" he was published a couple of times. His short-story contributions to Authonomy - "Harper Collins' online slush pile" - struck gold, however.

"One day I got an email saying an agent was interested in speaking to me, based on a couple of stories I'd posted. At first I thought yeah, right - this will be someone who'll get me to a vanity publisher for twenty grand or something. But I googled him and found he was a really good agent who represents all sorts of author as well as various celebrity memoirs."

In 2009 Bell brought out a book under his own name, but it failed to sell. "Eventually, though, I started work on the first three chapters of what would become The Killing Season. But my middle daughter was born, which threw me off the book for about a year.

"I went back to it from time to time. On reflection, it probably benefited from having a long gestation period. In 2012 I sent it to the agent. Orion, the publisher, liked it and asked me down to London.

"They asked me where I saw the series going. I'd invested a lot of time in making sure it could be a series: that was one thing I'd learned, that publishers really like series characters. I was forearmed in that sense."

Where did Mason Cross come from? "That was my agent's suggestion, that we go back out with a different name. We had submitted under my real name for the first book, but as commissioning editors tend to have quite a good memory for names, the agent thought it would be a good idea to come at it fresh without any preconceptions.

"The publishers also thought it would be good to have an American-sounding name for a book set in America. I had a few suggestions, none of which they liked, but I had a villain named Mason Cross in another book proposal, and my agent liked that. It's short, and fits on a book cover.

"The agent asked, 'How would you feel about being called Mason Cross?' I said they could call me Susan Cross if it means getting a book deal!

"I like that separation of identity - Mason Cross, Gavin Bell - although it can be confusing sometimes when I check into a hotel and don't know which name I have been booked under."

He's still chuffed that Child should have liked The Killing Season, which came out in April 2014, so much. "That was fantastic. A lot of the early reviewers said the book had reminded them of him.

"I'm a big fan of his - he was a big inspiration of mine, not just in the type of books he writes but also in the fact that he's a Brit [Child was born in Coventry] who writes American thrillers and seems to get away with it. That made it almost seem okay for me to try that."

He recently sent a copy of the book to Bill Clinton, who likes thrillers. To his surprise, Clinton replied with a letter of thanks saying: "It looks terrific and I can't wait to read it."

The Samaritan was actually written 18 months ago. The idea for the plot occurred to him in a way that recalled something Stephen King had said in On Writing: the most interesting situations can usually be expressed as a What-if question.

"You sometimes hear about writers getting an idea for a book in one go," Bell says. "That had never happened to me - it was always a more painstaking task, I guess.

"But I was driving late one night, a couple of years ago. I was driving over a hill when I saw a car that had broken down by the side of the road,.

"There was another car parked nearby. In my headlights I saw a woman waiting as a man changed her front tyre. I thought 'Good Samaritan', as you do. Then I thought, But what if it was a Bad Samaritan, helping that woman in a dark and isolated spot?

"I remember thinking, surely someone will have done that before as a serial killer's MO. I dug around but couldn't find a book called The Samaritan. It struck me as being a really cool hook for a book.

"It just happened that it fit in really well with Carter Blake's back-story, the fact he used to do something different, back in the day, and that it's always interesting to see ghosts from the past coming back, to cause trouble.

"I keep a notebook in the car, not for epiphanies like that one, but because, when you're writing a book, it's when you're away from your computer that you can mentally work through any problems in the plot.

"That was something one of my old jobs - delivering pizzas in Hamilton - was great for. You'd be driving around highly familiar roads, not having to think too much about what you're doing, and your subconscious can really get to work on that."

Carter Blake's third adventure, Winterlong (named after a Neil Young song) "is more of a spy/conspiracy thriller, hopefully drawing on Hitchcock, and on authors like Robert Ludlum". A fourth one has been pencilled in for the year after next. "Intimidatingly," he observes, "it's already listed on Amazon for pre-order in 2017." He has yet to start work on it, though. All in good time.

Bell talks about the author's traditional pleasure of seeing one's name on the bookshelves, and mentions that he'd "love to write [graphic] comics" - he describes himself as a Batman fan on his Twitter bio.

In the meantime, he's got three young children (aged nine, five and two) to help raise, and a full-time IT job to hold down. But he's delighted with what he has achieved so far. "All I ever wanted to do," he says, "was just to get a book out there."

* The Samaritan, Orion Books, £13.99.