The title of Jane Shilling's The Stranger in the Mirror might sound more like an Agatha Christie whodunnit than a "memoir of middle age", but there's no doubt about the content. In 256 pages Shilling puts the ageing process under the microscope and, as we read, we squirm.

I've got a lot in common with Shilling. We are both fiftysomething, living in south London and mothers of only children (now at university). I, too, am bobbing about in the shallows of the menopause, feeling like I did when I was 14 and lagging behind in the puberty stakes. When will it happen and what is it anyway? The only certain thing is its inevitability; everything else remains shrouded in mystery.

As Shilling admits, there are no hard rules and hers is not a guidebook to dealing with the hot flush and the whiskery chin  there are plenty enough of them. This is a deeply personal, at times savagely raw account of one woman's realisation that she cannot be young for ever.

Shilling is a gorgeous writer and there are huge chunks of this book that I would happily steal. Her descriptions of the tedium of motherhood, her memories of grandmothers and her cousins' cast-off clothing took me back to my own past. Her description of a pair of Dolcis shoes is so vivid that I was surprised to look down and see I wasn't wearing them.

The good bits are so good that it's a shame when Shilling comes across as ever so slightly superior; sometimes you get the feeling that her menopause is more rarefied than anyone else's, that there is a more ordinary proletarian menopause in store for the average woman. That said, Shilling is fabulous on the fairytale quality of women's lives, the "be careful what you wish for" morality that freezes the over-Botoxed face into that of a gargoyle. Her literary roots are evident all the way through; she might write with the lightness and flow of a concert piano player, but it's backed up with academic muscle.

As a child she "borrowed" personalities from the books that she read, variously being all of the March sisters from Little Women, Pauline from Noel Streatfeild's Ballet Shoes and Bobbie from The Railway Children. On reading this, I was transported back to my childhood, when I spent a year of my life pretending to be Maddy from The Swish of the Curtain. Later, pregnant and single, she reaches for Margaret Drabble's The Millstone, finding comfort in Rosamund's similar predicament.

But when Shilling reaches middle-age, she runs out of heroines. "Where," she asks, "are the fiftysomething literary role models?" Some might argue that literature is stuffed with meddling middle-aged women, but Shilling is right that few take centre stage. Just like an awkward adolescent convinced that they are different from anyone else and that no one can truly understand what they are going through, Shilling finds it hard to figure out where she is meant to fit in. What sort of middle-aged woman should she be?

To be honest, I think most of us feel like this, adrift and rudderless, but whereas Shilling thinks that other women do little to help, I've found the opposite to be true. In fact, one of the few perks of getting older has been the sympathy of my sex: for the first time in life, I've felt a sense of sisterhood.

Shilling is forensic in her dissection of "this time of life"; she is merciless on her own failings and in her criticism of others (for example, she thinks the Grumpy Old Women series is threaded with self-loathing; I  ahem!  would beg to differ).

But although at times I felt like throwing the book at the wall, there were so many pages of beautiful writing, especially the chapters featuring her son, that by the end I came to the conclusion that if Jane Shilling feels that literature is short on fiftysomething heroines, then the best thing she can do is supply us with one. If this woman wrote a novel, I'd buy it in a heartbeat.

Jenny Eclair is currently on tour with standup show Old Dog, New Tricks. Her novel, Chin Up Britain, is out now

Jane Shilling's meditation on life as a middle-aged woman might have amounted to little more than a compendium of truisms, a kind of highbrow companion to the tabloid coverage of the trials and tribulations of Strictly Come Dancing's Arlene Phillips and Countryfile's Miriam O'Reilly. Her starting point is certainly familiar. "Throughout my adult life I had been accustomed to find my own experience as a woman reflected in the culture. Magazines and newspapers contained pictures of women of more or less my age, dressed in clothes that I might also like to wear, describing experiences that were familiar to me. Programmes on the television and radio took as their raw material the lives of my contemporaries. In bookshops, the female experience appeared in myriad narrative forms. Until the onset of middle age, when, all of a sudden, there was apparently no one like me at all. Like the children of Hamelin led away into the mountain cavern, we had all vanished."

"The Return of the Disappearance of the Middle-aged Woman": a story and a theme elaborated endlessly on TV and radio shows such as Grumpy Old Women and Woman's Hour, and in newspaper and magazine articles the whole western world over. Shilling may start in the same place as everyone else, but she goes in a very different direction. Where others turn to caricature and tend to simplification, Shilling turns in and on and over herself. The Stranger in the Mirror is true to its title. It makes the familiar strange.

The book announces itself as a memoir, but is more an extended essay  imagine Montaigne as a thoroughly modern unmarried mother and freelance journalist living in south London and recalling her post-pregnancy everted navel and the evenutal "sinister unknitting of the bony bonds of the pelvic girdle". The essay form, with its drifts and lurches, suits Shilling's purposes perfectly as she catalogues her experience of middle-aged confusion and loss. She looks at old family photos, remembering her grandmothers, Mrs Shilling and Mrs Charlton, who "seem quite clear about who they are".

She takes an inventory: "I had made neither myself nor anyone else happy; done no particular good and some real harm, squandered my chances of love, left my friendships untended, wasted time and opportunities. I had drifted aimlessly through the decades of my prime." She contemplates life with a teenager. "He is four inches taller than me. Under his bed are magazines in which girls with football breasts compete for space with news of Premiership transfers . . . The balance between us is shifting subtly: my arc of energy and authority just past its apex, curving imperceptibly towards a descent while his crosses it, rising." She concludes: "It is a long time since I have felt like the queen of the world."

She writes at length and diverges on "the accumulated small losses of middle age": the loss of beauty; the loss of dignity; the loss of parents; of children; "the proliferation of small mechanical failures". But best of all she writes about the loss of things you don't even realise you're losing. The loss of candour, for example. She remembers in her 20s and 30s talking with her friends and the way in which "we always used to report to each other from the front lines of our lives . . . The vagaries of our lovers, our employers, our parents, our shopping habits, our looks", all "part of a rolling comic monologue", a rolling monologue which slowly, gradually runs out of steam. "It took a while to notice, so long were the spaces of time between them, so uncommunicative the contents when they came  little more informative, mostly, than the births, deaths and marriages columns of the newspapers."

She touches on Der Rosenkavalier, Colette's novels, the Wife of Bath, Anita Brookner, Joanna Trollope, Anthony Trollope, Madame Bovary, TV doctor Hilary Jones, the clothing range in M&S. She explores the self-imposed roles of self-concealment, "cosmetic, sartorial, surgical, pharmaceutical". She dissects Germaine Greer.

She accuses Dr Robert Wilson's 1966 book Feminine Forever of converting middle age "from a stage of life rich in ambiguity and nuance into a curable medical condition". She explains how and why she has used clothes to make up stories about herself, as "fantastical and mendacious as possible", and why she therefore scorns the accepted codes of middle-aged female dress: "Cashmere cardis, crisp white shirts, the essential black trouser, the perennial trench, good shoes, good bags, a string of pearls, a bright silk scarf or funky bangle to add a jolt of colour."

Everywhere there is detail, and nuance, and enthusiasm, and care  about others, and about words. Here is her description of her neighbour, Violet: "Her evenings were spent at the Bingo, to which she set off around teatime, dressed in vivid costumes of peach, turquoise or cyclamen with co-ordinated shoes and bag, heavily bejewelled earlobes clinking and knuckles clotted with golden rings, her lipstick matching her manicured nails ('filbert nails', she said, stretching them out for me to admire)." The Stranger in the Mirror: like filbert nails, stretched out for us to admire.

Ian Sansom's Mobile Library series is published by Harper Perennial.