For the relatives of the 270 people who died in December 1988 as a result of the bombing of Pan Am 103, this book will make for difficult reading.

For more than 23 years they have had to deal with their grief while wrangling with the stories of politicians, spooks, government agencies and conspiracy theorists. For others, it may well provide compulsive reading.

The only man convicted of the worst terrorist atrocity to have taken place on mainland Britain did not take the stand at his trial and consequently very few have heard his side of the story. There are many who will not want to, many for whom it may be too painful or too difficult to change views strongly held for more than two decades.

Rather than tell the whole story in the first person, the book is split between the formal revelations of the author John Ashton – a former member of Megrahi's defence team – and the italicised first-person views of Abdelbaset Ali Mohmed al Megrahi, the Libyan convicted of the bombing, who spent eight years imprisoned in Scotland.

For someone like me, who has covered Lockerbie for more than a decade, and met Megrahi and his family, to hear the details of his life, to be able to fill some of the gaps and answer questions that even his greatest supporters found uncomfortable makes this book compelling. For others the fact he is still alive, two and a half years after he was diagnosed with terminal cancer and released on compassionate grounds, and has been allowed to express his views, may prove sickening.

In Megrahi's preface he explains that the decision to abandon his appeal was a "terrible choice" to have had to make. He writes: "From the moment I made that decision, I was determined that, if I could not be judged in a court of law, then I should be judged in the court of public opinion." He says the relatives of the victims have his "utmost sympathy" and asks that they judge him with their heads as well as their hearts.

Much later in the book he reveals that he felt compelled to drop his appeal as he gained the impression from Scottish ministers that if he wanted to go home, he had no real choice. Legally, to be released on compassionate grounds, there was no requirement to drop the appeal. However, he describes the former Libyan ambassador and foreign minister Abdul Ati al-Obeidi being taken aside by Scottish Justice Secretary Kenny MacAskill.

He writes: "Obeidi said that, towards the end of the meeting, MacAskill had asked to speak to him in private. Once the others had withdrawn, MacAskill told him it would be easier for him to grant compassionate release if I dropped my appeal. He said he was not demanding that I do so, but the message seemed to me to be clear. I was legally entitled to continue the appeal, but I could not risk doing so."

Such interference was vehemently denied by the Scottish Government then and is denied now.

In the opening chapters Megrahi reveals less than flattering details – how he lied to his wife, for example, because he was travelling abroad a great deal to import different embargoed goods – illegal under the US trade sanctions at the time. And why he was in Malta on a false passport – a fact used against him in the trial to impugn his innocence. He says that, for those involved in importing embargoed goods, it was commonplace at that time to use a coded passport. He points out too that, if he were a terrorist, he was a very bad one. He held on to this coded passport for 10 years and willingly handed it over to the trial at Camp Zeist, such was his belief that he was and would be found innocent. The coded passport has repeatedly been used by the Crown as evidence of his guilt and his role as a security agent – something the book denies.

As the surface is peeled back from each of the key tenets of the case – witnesses and forensics – Megrahi gives his own version of events. Through his words the reader is given an insight into what he was doing when he heard about the indictment against him and then, towards the end of the book, what happened when he dropped his appeal and was granted compassionate release.

One of the weaknesses of the book is that we never really get to see inside his mind or his feelings. When he learns he is going home he writes: "The enormity of the relief that I experienced is impossible to describe. For 18 years I had been the victim of politics. Now, finally, I was its beneficiary." There are moments of greater poignancy, as when he sees his parents for the first time. "It was so incredible to see them all again," he writes, "that I had to ask one of the children to slap my face to ensure I wasn't dreaming." But, on the whole, the book lacks any illumination of his inner thoughts and emotional state.

Ashton presents layer upon layer of a circumstantial case which points towards the guilt of Iran and the Palestinian terror group, the PFLP-GC. Every point is carefully referenced back to police statements, precognition statements, security documents and critical information unearthed by the Scottish Criminal Cases Review Commission (SCCRC). It was, critically, the SCCRC which referred the case back for a fresh appeal in June 2007 on six different grounds. A summary of these was presented at the time but the details of the 800-page report have never been published despite efforts by campaigners and the Scottish Government.

Ashton had access to the full report and draws upon it throughout each chapter, referring to new statements and documents unearthed by the SCCRC, which point to major inconsistencies in the evidence of key witnesses such as Tony Gauci – the shopkeeper who claimed to have sold Megrahi the clothes found in the suitcase thought to have housed the bomb – plus holes in the forensics arguments.

Gradually Ashton pulls together a palimpsest of the Lockerbie story, scraping away at the original prosecution case, revealing how key witnesses were compromised by reward money and their roles as security or double agents, and how the Scots police were often undermined by the American security services.

The book states: "Being in the dark was, by then, a familiar experience for the Scottish police. It was not until September 1990 that they finally learned of the existence of Mebo [the Swiss company the prosecution claimed sold the timers to the Libyans], having once again been spoonfed information by the intelligence services - the day before the [Scots police] visit, CIA officers met with the Swiss police and intelligence services."

It is the author's encyclopedic unpackaging of all the key elements of the prosecution case, new evidence, previously undisclosed documents and the grounds of the SCCRC that make this the definitive Lockerbie account. Undoubtedly there is much that will be contested by the Crown, and there are many who will not want to read of the possibility that the only man convicted of this terrible atrocity may not in fact have been responsible. Certainly, the book indicates that, had the second appeal taken place, Megrahi would have been acquitted. Whether he is guilty or not is up to the reader to decide.

Megrahi – You Are My Jury: The Lockerbie Evidence

John Ashton

Birlinn, £14.99