It is nearly a decade since Sally Lane received the phone call that changed her life for ever. It came from her elder son, Jack Letts, then 18, who had been travelling in Jordan on a gap-year trip from their home in Oxford. “Mum, I’m in Syria,” Jack said, beginning a chain of events that saw Lane lose her home and her job and, for a time, her freedom. Jack was calling from Raqqa, where he had gone in search of a Muslim caliphate he had fatefully, adolescently, come to idealise, before the horrors of Islamic State were known to the world.

“It is difficult to describe the terror of knowing your child is in the most dangerous place on earth and there is nothing you can do about it,” Lane writes in her frank and clear-eyed account of the years that followed. Her book is a tale of survival and an account of the legal process that consumed her and her husband, John Letts, in the name of counter-terrorism. The thread that runs through the story is Lane’s maternal refusal to give up on a son who became known as a pariah across the world (“Jihadi Jack” as he was christened in 2016 by a Sunday Times journalist) and who was subsequently stripped of citizenship by a government more concerned with headlines than justice. A son who remains to this day in a Kurdish prison, long convicted in the public mind, with scant evidence, of association with crimes against humanity.

To begin with, Lane retraces the steps that led her family into that nightmare, steps that she has gone over and over in her head since that first phone call: Jack’s hyperactive infancy; his precocious outrage at any injustice; his sudden teenage disaffection at school and the obsessive compulsive habits that led him into radicalisation and sympathy with Islamic fundamentalism. Lane’s painful, painstaking reconstruction of that path also leads her to all the questions any parent would ask, questions for ever levelled at her on social media: what effect did her rocky relationship with Jack’s father have? Were they too indulgent? Too naive? Too liberal? Where did they lose him?

If Lane’s night-terrors about Jack’s chosen fate were not enough, she and John were persecuted and eventually prosecuted for their efforts to save their son. A £6m investigation – “Operation Kilojoule” – into their limited interactions with Jack after he was trapped, willingly and then unwillingly behind IS lines, saw them brought to trial at the Old Bailey in 2019, for “funding terrorism”. Their crime? Sending – against conflicting official advice – a little more than £200 in 2015 to try to get Jack out of Syria (and for him to replace a pair of broken glasses).

I sat through some of that trial and subsequently wrote about Lane and Letts. It was hard not to conclude that they had been used by the Prevent programme as extreme evidence of its “even handedness”. If this white, liberal, middle-class couple – he was a heritage wheat farmer, she worked for development charities and the NHS – could be in the dock, how could the anti-terror programme be criticised for unfairly targeting British Muslims?

Lane details the consequences of this process as it spread through their lives: first they were abandoned by fearful friends and colleagues, then John lost his business, then their bank accounts and all assets were frozen, their computers and phones seized and their search history deconstructed. They were trashed on social media, targeted by hate groups. One judge locked them up and another bailed them in the belief that they were “two perfectly decent people who have ended up in custody because of love of their child”.

And all the time that child became more horrifically impossible to reach. Lane and Letts researched cultish brainwashing, interrogated Jack’s mentors and friends. They pored over scraps of information from journalists and charities and riddling and condemnatory media posts from their son, who consistently denied he had been a fighter for Islamic State. Letts and Lane, who had joint British and Canadian citizenship, were stonewalled by successive foreign ministers, chasing down redacted minutes of Kafkaesque meetings; they discovered that they were grandparents (Sally knitted a blanket); that Jack appeared to despise them as non-believers. They heard him sound nothing like his old self – referencing martyrdom and beheadings – and then exactly like it. In 2017, hope that he might return home to face justice emerged after Raqqa fell and he escaped alive; it was dashed when they discovered he was being held in a cell built for five people with 30 others; that he spent months in a solitary space not large enough for him to lie down. In that period the UK government allowed at least 400 IS volunteers to come back, often without sanction, but decreed that Jack and Shamima Begum – the two prisoners who had made the most tabloid headlines – must lose their British citizenship.

Lane – whose story continues to attract more judgment than sympathy – records every circle of despair and false hope with heartfelt candour. After her relationship with John broke down, she moved to Ottawa – on a cargo ship because she is on a no-fly list – where she continues to campaign for her son to be repatriated in Canada. Her book is a compelling expression of that determination – one that awaits its final chapter.

• Reasonable Cause to Suspect: A Mother’s Ordeal to Save Her Son from a Kurdish Prison by Sally Lane is published by Dundurn (£15.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply