“We have two lives,” Dr Matt Morgan writes, before clarifying: “The second begins when you realise you have [only] one.” Sometimes, as the case studies in this book detail, this realisation comes more suddenly and profoundly than most of us can imagine. For more than 20 years, Morgan has been a specialist doctor in intensive care, labouring at the extreme margins of life. Just occasionally, in his day-to-day education in human mortality, he has witnessed what might, in other traditions, be thought of as supernatural events: people whose vital signs have flatlined, but who have returned to tell the tale. The stories in this book – a sequel to his bestselling Critical – are his accounts of those impossible second acts, and his reflections on what we can learn from those lucky few who have experienced both possible answers to the question of “to be or not to be”.

The “deaths” Morgan examines here come in several shapes and sizes. Ed, now 47, was “fatally” struck by lightning at 17 (and had to overcome his guilt at his best friend not being so fortunate); Luca, 30, lost a battle with Covid during the pandemic, but was restored by the blood oxygenation technique ECMO; Summer took her own life at 25, and regretted it even as her breathing petered out; Roberto was frozen solid on a mountain ledge in the Dolomites and did not register a heartbeat for eight hours and 42 minutes before a flicker returned; the former Welsh rugby international Rhys Thomas, after a catastrophic heart attack, has lived for 11 years without a heart at all – an artificial alternative giving him a lease on life while he awaits a transplant.

The news each of these survivors brings from that “undiscovered country… [from which] no traveller returns” is, in individual ways, a kind of double gratitude. First at the unlikely chance and scientific wonder of their deliverance; second at the privilege of their unique knowledge – that never to be forgotten understanding of the once-in-a-lifetime joy of consciousness itself.

Morgan not only tries to inhabit some of this carpe diem wisdom, but to find lasting ways to impart it to his reader (the lessons of the book are both medical parables and memento mori of one sort or another). Describing Roberto’s remarkable revival through hypothermia, for example, Morgan speculates on the theory that another miraculous return, two millennia ago, may have been the result of a comparable phenomenon: “hypothermia induced by crucifixion may have simulated Jesus’s death. His resurrection [might actually have been] through gradual rewarming in a cave with a consistent temperature. Forget the divine intervention; it was more like divine insulation.”

If Morgan is sceptical about acts of gods, he is never complacent about the special magic of lives lived to the full. He takes as his first example of this his wonderful Welsh Aunty Win, whose funeral he attends on his birthday, and whose 97 years he celebrates in a heartfelt eulogy, testament to her “5,044 Saturdays and lazy Sundays, 1,164 bright full moons, six dark solar eclipses, seven houses, five jobs, two proposals. Three billion heartbeats.” It was Win’s example that caused him to start keeping notes that make up this book, about people who came, in far more dramatic circumstances, to her innate understanding that life was for living.

Inevitably, you learn things along the way here that might act as the handiest set of new year resolutions. The ongoing imperative in every life to find meaning; the solace of being in nature; the responsibility not to waste time in bitterness and to always find space for those you love; the necessity of breaking destructive habits and addictions, and to do all you can to inculcate positive alternatives. YOLO – you only live once – may be the overriding message of these survivors’ tales, but that doesn’t mean “splurging your savings, or making bad choices you might regret the next morning… it means finding meaning in the little things that matter. It means being open to new experiences, whether it is swimming in the cold sea or learning to paint, or learning the kazoo,” Morgan writes. “Go on. Go nuts.”

Summer’s near suicide prompts several of these revelatory moments. The first of those truisms is that mundane fact that the ultimate irreversible decision is not always the result of long despair, but of fleeting panic: “Several peer-reviewed research papers have shown that 70% of [suicide] survivors thought about killing themselves for less than half an hour, with a quarter considering it for just five minutes,” Morgan writes. Summer’s subsequent reflections reinforce that tragic understanding; she is among those here who experienced seeing life flash before their mind’s eye as her heart stopped, and her single insistent thought at that moment was this: “People are important, not things.”

Summer’s second life – sustained with the help of eye movement desensitisation and reprocessing (EMDR) and judicious immersion in the video game Tetris – has been an ongoing lesson in mindfulness: “I’m trying to live for the next few moments, not too many more,” she tells Morgan, before delivering that hard-won truth: “The opposite of happiness is not failure, but boredom.”

Morgan proves an excellent guide to such wisdom. He is grateful to bear witness to these stories – and self-effacing about the part he plays in enabling some of them. In the concluding chapter, he attempts to coalesce all of that thinking, to see if it can be ingrained in less life-threatening ways, by staging a collective “living funeral” for friends and colleagues – “eight grown men” – with different experiences of love and loss. At a remote cottage they get to hear what people may say about them after they are gone, and get to think hard about their legacy, about the lives they have touched, about the difference they have made. And then, a little like the survivors Morgan learns from, they get a chance to have another go at it for real.

• A Second Act: What Nearly Dying Teaches Us About Really Living by Dr Matt Morgan is published by Simon & Schuster (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply