FICTION:Don't be put off by the girlie cover: Keyes's brilliant new book, about a PI who suffers from depression, highlights her ability to make readers laugh about very dark subjects

The Mystery of Mercy Close By Marian Keyes, Michael Joseph, 506pp. €18.99

IF YOU WERE to ignore all traditional advice and judge the books of Marian Keyes by their covers, you’d probably think they were, to put it bluntly, fluffy. Or cosy. Ever since the publication of her debut novel, Watermelon, in 1995, the covers of Keyes’s novels have tended to feature pictures of shoes, or angel wings, or flowers, or butterflies.

But, as anyone who’s read a Keyes book knows, appearances can be deceptive. Those pastel covers concealed stories about addiction, infertility, depression and domestic violence, tales of bereavement and jealousy and serious illness. The books were also, as it happens, often very funny, but they were certainly not cosy.

And the same goes for Helen Walsh, the heroine of Keyes’s brilliant new novel. Having already made memorable appearances in Keyes’s earlier books about the Walsh sisters, she finally gets the chance to shine in The Mystery of Mercy Close. Or, rather, the chance to glower. Helen is the youngest and grumpiest Walsh, and when last seen, in Anybody Out There (2006), she had started a private detective agency. Six years and one recession later, things aren’t going well: as she says, “private investigators are luxury items and me and the It bags came out of things very badly”.

All her work has dried up, she’s lost her beloved flat and she’s had to move back in with her parents in the suburbs. Which means that when her ex-boyfriend Jay turns up asking for her help in finding Wayne Diffney, a member of the boyband Laddz who has disappeared from his house in Mercy Close just a few days before the group’s big comeback concert , she reluctantly agrees. She hates Jay, but there’s a lot riding on this huge event, and the money is good.

And, besides, there’s not very much that Helen doesn’t hate. Irritable, prickly and capable of holding a deeply felt grudge against a child, Helen is a gloriously grumpy creation. When anything annoys her, she adds it to her Shovel List. “It’s more of a conceptual thing,” she explains. “It’s a list of all the people and things I hate so much I want to hit them in the face with a shovel.” There are many, many things on the Shovel List, including hot drinks, music (not just specific genres but music in general) and any sort of “spiritual” book or CD. Commercial women’s fiction is a more complex field than many of its detractors claim, but, even so, Helen is not a typical heroine of the genre. Not least because she’s living with severe depression.

Keyes has written movingly about her own experience of mental illness, and in this novel she has written the most evocative depiction of serious depression I’ve ever read. Anyone who has ever struggled to understand what a person with depression is going through will find this novel revelatory.

Keyes shows brilliantly that it’s a matter not simply a matter of feeling sad but of feeling wrong: “Everything looked ugly and pointy and strange, and I felt like I was living in a science fiction film,” says Helen. “Like I’d crashlanded into a body that was similar to mine and on to a planet that was similar to earth, but everything was malign and sinister. I felt like all the people around me had been replaced with doppelgangers. I felt very, very not safe. Uneasy was the most accurate description of how I felt, uneasy to the power of a million.”

In the hands of a less skilled and subtle writer, Helen would get help for her depression, make a full recovery and live happily ever after. But, as the book makes painfully clear, sometimes the help doesn’t work. Sometimes a person can work her way through a serious depressive episode and emerge as a slightly different, slightly damaged person, a person who may eventually fall into that deep hole again and find it even harder to get out.

And sometimes she won’t get through it at all. “People get sick and sometimes they get better and sometimes they don’t,” says Helen. “And it doesn’t matter if the sickness is cancer or if it’s depression. Sometimes the drugs work and sometimes they don’t. Sometimes the drugs work for a while and then they stop . . . Sometimes you wonder if no outside interference makes any difference at all, if an illness is like a storm, if it simply has to run its course and, at the end of it, depending on how robust you are, you will be alive. Or you will be dead.”

The Mystery of Mercy Close doesn’t shy away from the grim reality of depression, though it’s anything but bleak. Keyes’s greatest strength has always been her ability to write very funny books about very dark subjects without any jarring shifts in tone, and this novel is no exception. Helen’s narrative voice is brilliantly consistent – her unsentimental account of her illness sometimes breaks the reader’s heart and her attitude to her romance with single father Artie is sometimes frustrating, but she remains wonderfully waspish throughout. And over the course of the novel her Shovel List expands to include such things as “active aging” (she doesn’t approve of the over-60s engaging in exciting activities when they should be staring out of their windows, observing things that will later be useful to an investigating private detective).

But amusing as Helen’s grumpiness is, her mother, known to all as Mammy Walsh, is responsible for some of the the book’s funniest moments. When she starts speaking in an unusually sassy way, Helen asks: “Why are you talking like that? What shows have you been watching?” To which Mammy Walsh replies: “Ah, you know, the usual. America’s Next Top Model. Whatever is on.”

There’s also a delicious streak of satire in Keyes’s picture of modern Ireland, particularly the affectations of the middle classes. (Helen and Wayne share an affinity for the Farrow and Ball-esque paint brand Holy Basil, whose colour chart includes paints called Gangrene, Quiet Desperation and Local Warlord.) And the book is also, rather wonderfully, a genuinely thrilling and well-constructed mystery. As Helen tries to find out the truth about Wayne’s disappearance, interrogating his neighbours, family and former bandmates, the reader becomes just as determined as she is to discover what happened to him. I hope Keyes writes about more of Helen’s investigations – not only is this surly detective too good a heroine to abandon after one book, but I’d love to see what else her creator could do with the crime genre.

I hope this book will prove to the sceptical that just because a book has a pretty cover doesn’t mean it’s not complex and intelligent. Recently, tube stations in London were adorned with posters for The Mystery of Mercy Close bearing the words “Relax, Marian’s Back”.

It’s snappy marketing, but it’s not quite accurate, because this isn’t a relaxing book. It’s an invigorating, spiky, thrilling, very funny and deeply moving one. And if people decide not to read it because the cover features girlie shoes and teapots and wine glasses, well, that’s their loss.

Anna Carey’s first book, The Real Rebecca, won the Senior Children’s Book of the Year prize at the 2011 Irish Book Awards. The sequel, Rebecca’s Rules, is published this month