An elegant woman enjoys a gin and tonic and dinner with her husband in a cosy Italian restaurant near their Greenwich Village apartment; their relaxed, lighthearted conversation, long familiar to them, doesn’t miss a beat. But as the time to leave this regular date draws near, he bids her farewell and she, in practised response, surreptitiously changes her dressy shoes to trainers so that she can rush home to arrive before he does. Once there, their dinner will be entirely forgotten to him and, in a painful reversal of their previous intimacy, he will ask her to leave – in the past he has evicted her to spend the night in the hallway or even called the police. He will not believe that they are married, and will react to evidence to the contrary – their shared belongings, anecdotes of their life together – as though it were planted or invented, a rotten fraud being perpetrated on him.

In Travellers to Unimaginable Lands, Dasha Kiper, a clinical psychologist who works with caregivers to people suffering from dementia, is primarily focused on Elizabeth, the wife; what this form of apparent rejection means to her, and how she is able – or not – to negotiate it. There is no story in this book that is not equally heartbreaking: whether it is quotidian, as in the mother who repeatedly removes items from the freezer despite her daughter’s dogged attempts to stop her with entreaties or taped-up instructions; or whether it is comparatively elaborate, as in the elderly woman who befriends the dead authors in photographs on the books she loves, going so far as to invite Stefan Zweig for dinner and ignoring her husband as she attempts, with no outward sign of success, to make conversation with him.

It is striking, though unsurprising, how many of these incidents revolve around the rituals of food, or other daily activities, such as shaving or showering. In the drama of delusion, home is the theatre, the meal table a frequent stage set, and the domestic scene comes freighted with memories and associations. Kiper works only with family members caring for those with Alzheimer’s or other types of dementia, rather than healthcare professionals, and is therefore highly attuned to the way that individual histories affect responses to this new, radically altered reality. Writing about “dementia blindness” – how long it takes relatives to recognise or acknowledge the new reality – she notes that “invariably, a caregiver’s most susceptible blind spot is an old familial wound”. Thus we learn how the freezer that has become a battleground is an echo of a mother’s previous attempts to control her daughter’s diet, and her daughter’s subsequent eating disorder. Those taped-up notes, Kiper observes, are a frequent sight in the dementia household, an attempt at order that is rarely effective.

Kiper marshalls her scientific evidence with extreme care to make the case that the “healthy” brain will inevitably founder when confronted with a mind that is fracturing. We default to making sense of what we see, to building relationships on the basis of a shared reality, to believing in an essential self that, even when threatened by disease and deterioration, persists. When, for example, a patient appears to understand perfectly well what is happening around them, or when they exhibit behaviour strongly suggestive of the personality we know, we are geared to fill in the gaps, to keep the show on the road; when that fails, we might become frightened, filled with grief and rejection, bewildered.

Travellers to Unimaginable Lands is a work of exceptional compassion. Kiper attempts to show caregivers that their reactions – of anger, frustration, disbelief, isolation, immense sadness – are not merely understandable but a function of their own brain’s operations. It is also a deeply imaginative response to that “unimaginable” territory which must, somehow, be navigated. Anyone on that journey will surely find this book immeasurably valuable.

• Travellers to Unimaginable Lands: Dementia, Carers and the Hidden Workings of the Mind by Dasha Kiper is published by Profile (£16.99). To support the Guardian and the Observer buy a copy at guardianbookshop.com. Delivery charges may apply.