There are many heartbreaking moments in this beautifully written book, but the first comes before it even begins. In a dedication to her son Louis, author Alison White says how she wanted to write it so that people would understand disability and caring, but also, “to be totally honest, I wanted to write something that would make people consider being Louis’s friend”. Beneath that simple plea lies the great fear of so many parents who nurse a severely disabled child through to adulthood: “What will happen when I’m gone?” It’s a measure of this unsentimental and clear-eyed account that White never labours this point, or any other of the myriad anxieties that accompany long-term caring. Instead, she just tells us what it’s like: and it is, in equal measure, admirable, uplifting, terrifying.

White’s story begins with Louis’s premature birth and an account of his time in an intensive care unit, where he comes close to death. When the story moves back to the later stages of White’s pregnancy and the catastrophic failure of a midwife to check White’s blood pressure at a vital moment, you are already primed to shout at the page: “Check her blood pressure, for God’s sake!” White goes into pre-eclampsia, a condition that can be fatal for mother and child, and Louis is later diagnosed with cerebral palsy.

As a society, we are fond of eulogising short-term heroism: the soldier or firefighter, bravery containable within a single story. The uncomfortable truth that Letter to Louis lays bare is that the heroism of long-term stamina, the daily caring over many years, is neither tidy nor anecdotal. At times, with two other children to care for and Louis waking five times a night, still undiagnosed and in constant pain and distress, White feels simple despair. “I picture the cliff. I picture jumping holding you tight in my arms, falling and falling through the air.”

Although there are moments of joy – when Louis first speaks, manages to walk a little – White offers no comforting platitudes. “My destiny has been decided. The realisation hits me full force in the stomach. I don’t want this destiny.” Very soon, you come to admire White’s fortitude not only in raising a child with a disability but in resisting the temptation to punch one or two of the many idiots she encounters along the way: notably a Clarks shoe shop assistant who refuses to sell Louis, a wheelchair user, a pair of shoes because he can’t walk across the room for her to check the fit, and an unspeakably unhelpful occupational therapist who won’t authorise the toileting equipment that he needs, insisting he has to learn to wipe his own bottom, even though it is a physical impossibility for him.

It is shocking to learn that at Louis’s most disabled, after a disastrous foot operation, the family are only entitled to two hours’ help a month. Respite care only comes, eventually, when they are close to breakdown. At one point, Louis has five different social workers in eight months – and then, unbelievably, faces the overnight withdrawal of all respite care when he reaches his 18th birthday.

Conversely, White explains the difference that the support of family and friends and small acts of kindness from strangers can make. When she takes Louis on a special day trip to London because he wants to travel on the tube, Transport for London staff take it upon themselves to radio ahead to each other to make sure there is always someone to help them at each station he wants to visit. Once in a while, a therapist or doctor actually listens to White’s knowledge and expertise regarding her son’s condition.

Eventually, though, the devastating long-term consequences of constant sleep deprivation and caring take their toll. A slightly mysterious chapter towards the end of the book sees the author disappearing to Edinburgh to walk and sit in cafes: it’s never stated overtly whether this is for work or because she has had a breakdown or a combination of the two. The wonder is only that it has taken so long.

This chapter is just one of the half-told stories in this book: the author is mindful of the need to protect the privacy of her husband, although it is clear their marriage has come close to breaking point many times, as anybody’s would, and that of her other two children. Beneath it all is a cold fury that any decent-minded reader will share towards a society that fails to understand that unless carers are properly cared for by the rest of us, it all unravels.

Above all, this book is a plea for understanding, for the rest of society to pause a while when they encounter someone like Louis or his parents. The huge difference a kindly word or helpful act can make – and the devastation a thoughtless act can cause – cannot be underestimated. This book is an essential read for anyone who has ever moaned about their taxes going to pay for disability services: it should be legally required reading for anyone in the medical profession or anyone with the power to decide about cuts to those services. The rest of us should read it for an acute insight into just how lucky we are.

• Letter to Louis by Alison White is published by Faber (£12.99). To order a copy for £11.04 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99