In 1980s Ireland, Brigid becomes obsessed with a photo of a statue of the Virgin Mary and things spiral devastatingly from there. Erica is a hardworking fashion journalist in her 20s who believes the contraceptive coil she has had fitted is actually a camera planted by MI5. James is training at Sandhurst when he finds himself on parade in his pyjamas.

Each of these vignettes forms part of an instructive case study in Nathan Filer’s intelligent, absorbing narrative exploration of schizophrenia, The Heartland: Finding and Losing Schizophrenia. Filer, you may recall, deservedly won just about every major fiction award in 2013 (including the Costa book of the year) for his debut novel, The Shock of the Fall, which told the story of a young man dealing with mental illness and grief. Both that novel, and this, his first full-length nonfiction book, are informed by his earlier career as a mental-health nurse.

At the beginning of The Heartland, he tells us that, although he never gave Matthew, the protagonist in The Shock of the Fall, a diagnosis, if he had it would have been schizophrenia. But schizophrenia (the “heartland” of psychiatry, as it used to be known, hence the title) doesn’t feature here. Instead, it is referred to as so-called schizophrenia throughout Filer’s 250-page attempt to get to grips with the history of the diagnosis, and what exactly it means. He uses “so-called” not as in the teen dismissiveness of the 1990s television show, My So-Called Life, but to reflect the paradoxically inherent shifts in the psychological and psychiatric landscapes.

But his scope is even broader than that. What is any mental illness diagnosis supposed to mean? And who gets to decide? And what happens then? Filer digs into ostensible causes and symptoms; stigma and language; past and current treatments. He wants to discover and articulate how a mental illness leaves its print on the people who live with it or in its proximation.

The book, then, is not the equivalent of a professor addressing a lecture theatre, which is what makes it so effortlessly readable. Instead, Filer is a player-manager. His selection of quotes, facts and detail had me raising my eyebrows in exclamation, circling a phrase in recognition, mentally filing away an intrigue or twitching in distress.

Erica describes the transient nature of her delusions as being like: “When you kind of sniff something in the air. Except it’s sniffing a thought.” As the historian James Harvey Robinson observed: “Most of our so-called reasoning consists in finding arguments for going on believing as we already do.”

The experts Filer chooses to speak to are some of the most interesting in the field of mental health in the UK (I can vouch, at least, for King’s College London professors Anthony David and Graham Thornicroft, having shared a stage with them).

Thornicroft has written: “Popular knowledge about mental illness is a potent cocktail of profound ignorance and pernicious information.” In trying to rectify that in this book, Filer doesn’t tend to come down categorically on any side, which is a good thing.

On the rattle and shake of the pillbox, for instance, Filer consults Joanna Moncrieff, who readers might recognise from Alastair Campbell’s BBC documentary on depression earlier this month. She is a critic of antidepressant use, whereas Filer isn’t anti-medication. But he does touch upon something that is more broadly covered in the likes of Ben Goldacre’s 2012 book Bad Pharma, which is that “psychopharmacological research is a dark art”. (I agree with this, too, despite being on varying medication for a decade).

Filer can be more forthright elsewhere, however. The “overrepresentation of black people in the mental health system” is, he rightly tells us, a euphemism for “institutional racism”. And he declares that The Diagnostic and Statistical Manual of Mental Disorders, which is tiresomely always referred to as psychiatry’s bible, should be “like holy scripture, taken with a pinch of salt”.

It seems obvious, too, that we should infer from the fatally tragic story of another case study in the book, Clare’s son, Joe, diagnosed with schizophrenia and discharged from hospital without a proper care plan in place, that he was failed by the system (as was Clare).

Occasionally, there are flashes of humour (as when Filer trashes the later films in the Jaws franchise). But it’s the fascinating nuggets of information he unearths that make this book so successful. (How social media is replacing religion as one of the most frequent sources of paranoid delusions, for example.) It is written with a gentleness that has both its subjects and readers in mind.

If I was going to be picky, I would make two criticisms: The Heartland is very UK- and US-focused (which Filer himself acknowledges). And the final segment of the book is probably less easy to follow than his earlier free-flowing style, unless you’re extremely weird like I am and well into the lateral occipital complex.

Still, The Heartland is a book that everyone should read. Even though, as Filer puts it at the end of five sections of his upsetting, entertaining and fascinating, research: “We have found that nothing in the world of mental health is uncontroversial.”

Hannah Jane Parkinson was named journalist of the year at the Mind media awards last year for her piece It’s Nothing Like a Broken Leg

• The Heartland: Finding and Losing Schizophrenia is published by Faber (£14.99). To order a copy go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99