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Rule of Threes: A British Detective Crime Thriller Series
A high-stakes British crime thriller where a protected witness vanishes and the detective hunting her is already being tracked.
In South Manchester, duty does not take days off.
Days before his wedding, DCI Rick Castle is pulled into the one case he cannot refuse—finding a missing protected witness who could finally bring down an untouchable Mancunian crime lord.
The trail leads him to the desolate Scottish Highlands and explodes into violence and conspiracy, revealing a suspected leak within the police. Cut off from support, and fearful he's being watched, Rick pushes on.
But the closer Rick gets to the witness, the closer his family gets to danger, turning his obsessive investigation into a fight he cannot walk away from—and a battle for survival he may not win.
If the case does not break him, they will go after the one thing that will.
Rule of Threes is Book 5 in the DCI Castle series. Perfect for readers who love British police procedurals, relentless detectives, and gritty, high-stakes crime thrillers.
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Praise for the DCI Castle
★ ★ ★ ★ ★ Richly drawn characters… in a world of moral shades of grey.
★ ★ ★ ★ ★ I felt like I was there, on the streets, in every room.
★ ★ ★ ★ ★ A gripping crime novel… with complex characters and a story that keeps you hooked.
★ ★ ★ ★ ★ A fresh take on the crime genre.
★ ★ ★ ★ ★ Not many writers create a detective with vulnerabilities beyond the usual cop tropes.
In South Manchester, duty does not take days off.
Days before his wedding, DCI Rick Castle is pulled into the one case he cannot refuse—finding a missing protected witness who could finally bring down an untouchable Mancunian crime lord.
The trail leads him to the desolate Scottish Highlands and explodes into violence and conspiracy, revealing a suspected leak within the police. Cut off from support, and fearful he's being watched, Rick pushes on.
But the closer Rick gets to the witness, the closer his family gets to danger, turning his obsessive investigation into a fight he cannot walk away from—and a battle for survival he may not win.
If the case does not break him, they will go after the one thing that will.
Rule of Threes is Book 5 in the DCI Castle series. Perfect for readers who love British police procedurals, relentless detectives, and gritty, high-stakes crime thrillers.
___________________________________________________________________
Praise for the DCI Castle
★ ★ ★ ★ ★ Richly drawn characters… in a world of moral shades of grey.
★ ★ ★ ★ ★ I felt like I was there, on the streets, in every room.
★ ★ ★ ★ ★ A gripping crime novel… with complex characters and a story that keeps you hooked.
★ ★ ★ ★ ★ A fresh take on the crime genre.
★ ★ ★ ★ ★ Not many writers create a detective with vulnerabilities beyond the usual cop tropes.
360 pages, Kindle Edition
Published June 30, 2026
About the author
James Ellson
12 books27 followersJames Ellson was a police officer in the UK for 15 years, starting in London and finishing as a Detective Inspector at Moss Side in Manchester. When he left the police he started writing, and has been writing ever since.
James’s debut novel The Trail was published in 2020, and the sequel Cold Dawn in August 2022.
He also writes short stories, and was runner up in the 2021 Bay Tales Christmas short story competition with Roisin's Christmas Party. His story The Translator was published in a 2022 anthology (To Serve, Protect, and Write) of former police officers, now writers.
James is a keen climber and mountaineer, and has visited Nepal many times. In 2004 he climbed 6,812 metre Ama Dablam, and in 2008 soloed the Matterhorn. He lives in the Peak District with his wife, and manages their smallholding, which includes bees and an orchard.
James’s debut novel The Trail was published in 2020, and the sequel Cold Dawn in August 2022.
He also writes short stories, and was runner up in the 2021 Bay Tales Christmas short story competition with Roisin's Christmas Party. His story The Translator was published in a 2022 anthology (To Serve, Protect, and Write) of former police officers, now writers.
James is a keen climber and mountaineer, and has visited Nepal many times. In 2004 he climbed 6,812 metre Ama Dablam, and in 2008 soloed the Matterhorn. He lives in the Peak District with his wife, and manages their smallholding, which includes bees and an orchard.
Ratings & Reviews
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Displaying 1 - 5 of 5 reviews
Review of advance copy
The Rule of Threes, the fifth book in James Ellson’s gripping DCI Castle series, is an explosive addition to the world of gritty, modern day crime.
Days before his wedding, rogue DCI Rick Castle takes a remote Scottish Highlands assignment he just can’t refuse. Rick is a senior detective full of pure grit, loyalty, and empathy, and his relentless determination to hunt down and destroy the worst perpetrators of violence makes this a truly fantastic, edge-of-your-seat read.
This story has absolutely everything. While Rick’s fierce drive anchors the dark narrative, who couldn’t love his fiancée, Maggie? Ellson expertly balances brutal present-day imagery and haunting pieces of the past with a sharp sense of humour and a flicker of hope for the future.
I absolutely raced through this book to a totally unexpected ending that's left me desperate for the next instalment. It is the perfect read if you like your crime fiction dark, raw, and led by a relatable detective you can't help but root for.
If I hadn't already decided to give this book five stars I'd have added a star for mentioning the Green Welly Stop and acknowledging that Jaffa Cakes are indeed biscuits!
My thanks to Booksirens and Vinci Books for the ARC. This is my own opinion.
Days before his wedding, rogue DCI Rick Castle takes a remote Scottish Highlands assignment he just can’t refuse. Rick is a senior detective full of pure grit, loyalty, and empathy, and his relentless determination to hunt down and destroy the worst perpetrators of violence makes this a truly fantastic, edge-of-your-seat read.
This story has absolutely everything. While Rick’s fierce drive anchors the dark narrative, who couldn’t love his fiancée, Maggie? Ellson expertly balances brutal present-day imagery and haunting pieces of the past with a sharp sense of humour and a flicker of hope for the future.
I absolutely raced through this book to a totally unexpected ending that's left me desperate for the next instalment. It is the perfect read if you like your crime fiction dark, raw, and led by a relatable detective you can't help but root for.
If I hadn't already decided to give this book five stars I'd have added a star for mentioning the Green Welly Stop and acknowledging that Jaffa Cakes are indeed biscuits!
My thanks to Booksirens and Vinci Books for the ARC. This is my own opinion.
June 28, 2026
British DCI Rick Castle is just days away from getting married when he’s drawn into a deadly conspiracy. A powerful crime lord discovers that a key witness in witness protection poses a serious threat to his criminal empire and will stop at nothing to eliminate the danger. Packed with a compelling cast of characters, gripping mystery, shocking murders, relentless action, and an expertly crafted storyline, this is a thrilling page-turner from beginning to end.
July 2, 2026
I really enjoyed this fast paced, gripping read set in Manchester. I love the character of DCI Castle, along with his soon to be wife Maggie, they are very relatable. The plot is dark and immersive and I was completely hooked from the opening chapter. It's very well written and left me wanting to find out what happens next for Rick and his team. Thanks to Net Galley for my ARC.
Review of advance copy received from NetGalley
I felt like I was there at the scene whilst reading this book. It was so well written and creative by James Ellson. I really enjoyed it. Book 5 of the DCI Castle series
A protected witness vanishes and the detective finding her is already being tracked! By who ? The closer the detective gets to the witness, his family gets into danger Just brilliant!
A protected witness vanishes and the detective finding her is already being tracked! By who ? The closer the detective gets to the witness, his family gets into danger Just brilliant!
Displaying 1 - 5 of 5 reviews