Depression is no pussycat: it is a black dog, a hellhound. It is not thin autumn light but dark sun and depthless fall. Because its estate is one of mute disenchantment, you lack even the mortal consolation of art. Art is a field alive with sun motes and glyphic rooks; depression is the severed ear.

That said, depression has become something of an oversubscribed pitch in the black economy of confessional writing; and the prospect of another young, gifted and rich 'depressive' come to tell us about hell on trust-funded earth did not excite my own meagre sero tonin to butcher-than-usual levels.

However, get past the icky profiles that have attended Andrew Solomon - over-medicated, over-mediated and over here: Prozac's doe-eyed poster boy - and you find an exemplary text. Solomon is one of those New Yorker-trained writers who can charm the peacocks onto the lawn with mere statistical ballast; he is acutely aware of the contradictions at every turn of his tale, and the investigative reporter in him overrides the easier impulse to self-dramatisation. His own pain serves as a conduit to wider quandaries, where too many writers remain hypnotised by the dark mirror of disclosure.

Another common problem with 'the literature' is that people too often have a position - pro-Prozac, anti-analysis - to push. Solomon maintains a sharply perplexed outlook: he needles away at depression as if it was some malign multinational - Melancholia, Inc - always getting away with unaccountable intrusions into innocent lives. There's an almost thriller-ish pulse as he sorts through competing theories: is the villain neuro-chemistry or misapplied nurture? Personal hubris or unbiddable fate? All the signs seem to point to it being as simple - depressingly simple - as plain bad luck.

Many of us carry a genetic predisposition to depression, but if we manage to avoid coarsening addictions and personal grief (and the new Lee Evans sitcom), then the blade may never fall. Alternatively, you may live the kind of life previously only available to Moorish princes, but one day find yourself - like Solomon - gaspingly inert, helplessly incontinent, cluelessly incoherent: 'depressed'.

Solomon's glisteningly privileged life (Upper East Side family wealth, Yale, Cambridge, improbably lovely homes on two continents, early acclaim) actually works to his narrative's advantage, providing a suitably operatic backdrop for his descent into depression's underworld. His inaugural breakdown was triggered by the loss of his mother, in what is a strikingly Oedipal tale. (Oedipus in analysis: 'very' New York.)

A certain airlessness can afflict the depressive memoir - leading the reader into unworthy thoughts about the author being all too self-obsessed, depression or no depression. Solomon is scrupulously inclusive - like a wise Narcotics Anonymous chairperson, who leads strangers straight to the most pertinent part of their story, the parts of speech that are both messily singular and chimingly common. The Noonday Demon is formidably well researched: Solomon has a particularly keen touch with quotations and the testimony of others, building up a rich polylogue where other writers have settled for stark midnight soliloquy.

If I can't join his chorus of approval for the prescription fix, it can't be said he doesn't consider all the other options. (He's tenderly amusing about the battier alternatives.) If you have not had the experience of feeling your previously inviolate 'I' rearranged by drugs, legal or otherwise, it's hard to describe the attendant ontological shock: finding that the merest chemical veto or kiss can wholly undo the 'who' that you think you are. This is Copernicus in inner space: 'you' revolve around your neural mood, and not vice versa; your whole history may come down to potluck increments of this or that synaptic cement. What is posed as excitingly dark allegory in Lacan and Derrida becomes a clammily immediate reality: 'I' is indeed another . . . and one written in the opaque inks of biochemistry.

In the wake of truly wretched depressive episodes, Solomon the patient is ready to accept the stabilising ties of psycho-pharmacology. Here lies the razor's edge - razor supplied by Occam. Do we accept calamity as a human given, or let medicine write us a new 'script' for refillable eudaemonism? Paradise repeat-prescribed? Lives freed of crippling absence, yes - but Prozac's smooth and sexless pact also disappears the dark cameos of Van Gogh, Artaud, Plath, Lowry, Cobain. Ineradicable sadness may be a tax we must accept for accessing the more distant, moon-dust regions of ourselves. Love, too, can be a disabling monstrosity - then should we eradicate love? Indeed, Solomon theorises some depression as a form of evicted love, and is brave enough to admit a further possibility - that depression may occasionally smuggle in functions as epiphanic as they are disabling. Many sufferers find that depression's parting trace is also a first step towards previously unimagined spiritual complexity. Downtime can be the dark angel that delivers Rilke's urgent message: you must change your life.

There are political questions here too - the current wonder-drug status of Prozac being a depressingly familiar spectre. (Cocaine and morphine both were likewise hailed as handfuls of hope.) Prozac - trialed and trailed as a last-hope remedy for degree-zero depressives - is now doled out to mildly alienated everyman and his literal dog. Why are we so rosily accepting of Prozac, while maintaining our idiot's 'war' on all non-corporate medicines? If I'm making it sound like Solomon ducks such questions, he doesn't. In fact, the best one can hope for is that The Noonday Demon becomes a lodestone work: its value - whatever ad hoc quibbles one may have with Solomon's analysis - is that it takes what is a depressingly familiar tale, and makes it speak lessons of far wider import.

When you are truly depressed, eloquence counts for nothing: your mind has more in common with 5am TV static than the prose of Bernhardt or Beckett. That Solomon has shaped a richly eloquent testament from his own seasons in hell kindles something like hope in this unhappy reader.