JIM McCaul remembers all their faces: the young boy who blew off his jaw playing with a gun; an elderly man sliced open with an angle-grinder while cutting a cat flap; a woman who suffered serious head injuries after being crushed beneath a car.

Faces ravaged by cancer, marred through violence and left unrecognisable after industrial accidents. Over the past two decades Jim – a maxillofacial/head and neck consultant surgeon at the Queen Elizabeth Hospital in Glasgow – has painstakingly put them back together again.

He has written a memoir, Face to Face, which dissects his life and work. The pages conjure up powerful imagery which sears the mind's eye with stories of tragedy, loss, hope, life and death.

Jim knew by his seventh birthday he wanted to become a surgeon. During his early childhood in Linwood, Renfrewshire, he witnessed the chronic pain that his polio-stricken mother suffered, recalling how she would wince in agony due to the constant burning ache in her leg.

In the evenings, he and his younger brother Vincent would take turns to massage her knee in the hope of alleviating some discomfort. "I remember thinking that if I learned how to be a surgeon, I would be able to make my mum's pain go away," says Jim.

"I did have this romantic notion as a wee boy of dark green cotton drapes and surgical gowns. I thought: 'What a good job that must be.' The idea of helping people with my hands is still appealing. It is extremely rewarding."

Jim, now 49, studied dentistry and then medicine at Glasgow University – a maxillofacial surgeon must be qualified in both – before going on to do surgical training and hone his skills in hospitals across the west of Scotland, London and Florida.

It included a six-week stint at Jackson Memorial Hospital and Coral Gables Hospital, both in Miami, during the mid-1990s.

Having grown up on the outskirts of Glasgow, Jim was familiar with stories of gang violence. Although that largely involved knife crime. He was about to see up close the life-shattering impact of guns.

The rate of violent crime in Miami was almost three times the US national average. Jim assisted on operations including a young boy who blew off his own jaw after accidentally firing a handgun and a woman shot in the back of the head as she fled a hold-up at a drive-through ATM.

"The gun crime was staggering," he says. "I dealt with cases when people were first brought in and others who were in various stages of reconstruction. The fresh cases depended on you being on-call for trauma and you couldn't do that every night of the week because it was exhausting."

Jim doesn't mince his words when outlining the destruction that guns can cause. "It is hellish," he says. "In the movies, it is 'bang!' and you're dead. But the reality is it takes a while to die. People don't just fall down. They keep running for a while and then they slowly collapse.

"There isn't a big 'oof' when you are shot by a high velocity bullet. The momentum doesn't knock you on your back; it rips your insides. That is absolutely chilling."

As a consultant surgeon Jim spent eight years working at Bradford Teaching Hospitals in West Yorkshire before moving to the Royal Marsden Hospital and Northwick Park Hospital, both London, in 2014.

He returned to Glasgow last year to take up his current role at the Queen Elizabeth University Hospital.

Around 80 per cent of Jim's work involves treating patients with tumours of the face, mouth or neck. Aggressive cancers can pulverise jawbones, decimate tongues and throats, leaving those affected with scar-ridden, often unrecognisable appearances.

It is Jim's job to reverse or repair this damage. His main method involves what is called a "free flap transfer" where a piece of human tissue – skin, flesh, muscle or bone – is cut from one site on the body and transplanted to another to remedy a trauma, excised tumour or congenital defect.

Bone from the shoulder blade, hip or fibula in the lower leg can be used to reconstruct the jaw, while skin and soft tissue from the arm or thigh can form a new tongue.

One of the best parts of the job, he says, is making the telephone call to a patient's next-of-kin to tell them that surgery has gone successfully. The worst is when he must stand by and do nothing.

"I do a lot of cancer work and one of the biggest challenges is when you have someone who might not be quite fit enough to go through the whole procedure," says Jim. "That can be an awfully hard decision to make."

Face to Face: True Stories of Life, Death and Transformation from My Career as a Facial Surgeon by Jim McCaul is published by Bantam Press, priced £16.99