Practising barrister Imran Mahmood's debut novel, You Don't know Me (2017) was longlisted for both the Theakston Crime Novel of the year and the CWA Gold Dagger and is currently being adapted as a four-part TV series.
The novel was praised for the authentic voice of the main character, a young black man accused of murder, set against a background of gang crime in London..
In I Know What I Saw, Mahmood creates another convincing, disturbing character in Xander Shute, who has been living on the streets of London for over 30 years.
Once an investment banker, he left his old life behind after the suicide of his younger brother, Rory, and the collapse of his relationship with the love of his life, Grace.
One cold night, after a fight with another homeless man in Hyde Park, Xander finds the door to a Mayfair flat open and, cold and injured, takes shelter for the night.
He's woken from sleep as the owners return and, hidden behind a sofa, witnesses a man in a drunken rage murder a woman.
When he's taken into custody for the incident in the park, he tells the police what he saw in the flat, describing the scene in some detail.
However, when the police check the flat, there is no body and the décor of the flat is completely different.
Xander becomes completely obsessed, knowing he could have prevented the murder:
"I let her life be taken.
"The heaviness of that realisation crests over me . . . Her face presses up against my skull...
"I need to know more about who she is- was. And who killed her".
If he can find evidence that there was a crime, perhaps the police will believe him when he tells them again, "I know what I saw".
Xander knows, however, that his years on the streets have affected his mind and especially his memory, that all his memories "are fragments of something visual, pulled and glued together with my own brush.
"I can't distinguish the truth from the patches I manufacture".
As he tries to navigate the haze surrounding the present, he remembers his past.
This includes his difficult academic parents, Rory, his genius brother and Grace, meeting her at Cambridge and the life they had together before his jealousy drove her away.
I Know What I Saw is a complex crime novel, full of surprising twists and turns.
At the same time, it is an intriguing exploration of the mystery of memories, with constant references to Proust and the madeleines from A La recherché du temps perdu. It's an addictive read.
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