As the psychotherapist Philippa Perry has shown, graphic novels can educate as well as entertain: her recent book Couch Fiction is funny and irreverent, but it also answers many of the questions people have about what therapy is, and how it works. Darryl Cunningham's Psychiatric Tales is a darker beast than Perry's book: its author was formerly a healthcare worker in an acute psychiatric ward, and his experiences with patients, albeit heavily disguised, form the spine of this volume. But it is equally instructive. Until I read it, I did not know that while male schizophrenics often begin to show psychotic symptoms in their late teens, women patients are likely to start having delusions only in their mid-20s and early 30s (though the author doesn't speculate as to why). Nor was I aware how great the risk of suicide is among people who self-harm. Most strikingly of all, it had never occurred to me that, as Cunningham cheerily points out, we live in a society in which some psychopathic traits  selfishness, superficiality, an ability to manipulate others  are deemed to be nothing out of the ordinary.

Cunningham's descriptions of illnesses such as bipolar disorder and schizophrenia are pretty bald, and you should come to his book expecting a lot of blood, sweat and tears. His drawings, too, are stark: he works in black and white, and his style is wilfully naive. Some of his faces consist only of two lines: the determined curve of cheek and jaw; a claw-like nose connected to a pair of narrow, unreadable eyes.

The severity of these frames  light always fighting shadow  is deliberate. It's Cunningham's point that mental illness can strike anyone at any time: he presents it as a crowding-in, a darkness that falls inexorably, like the night. Realising, perhaps, that this is a doomy kind of a message, he also includes a chapter on famous depressives  Churchill, Nick Drake, Spike Milligan  which he calls "People with mental illness enrich our lives". To me, though, this felt like whistling in a lonely underpass. I would rather be content than creative, and I expect that Nick Drake felt exactly the same way.

Psychiatric Tales has a poignant coda. Thanks to his own mental health problems, Cunningham never completed his training to be a psychiatric nurse. But he was eventually saved by Prozac and his cartooning, which he promoted at first on the internet; it was the enthusiastic response online that encouraged him to complete Psychiatric Tales, a book he thought he'd abandoned. And now it has been published by Blank Slate Books, a small publisher with a big heart which has ambitions to be the Drawn & Quarterly of the UK (D&Q is a leading Canadian publisher of graphic novels). Last year, Blank Slate received its first order from the high street behemoth Waterstone's. Handily, its website links to Cunningham's blog, where he reveals that he is now working with people suffering from dementia. Meanwhile, he is also trying to finish his second book, The Moon Hoax. For Cunningham, if not for all of his patients, there is light at the end of the tunnel.