WHY WRITE a book on depression? Well, one of the oddest, and most oddly entertaining, books in English literature is Robert Burton's The Anatomy of Melancholy (1620), and now comes Andrew Solomon's superb new study. It's a fair bet that many readers of this newspaper will have had a brush with at least mild depression. It seems to be endemic to our way of life. Solomon writes that it is the acute awareness of transience and limitation in our lives that constitutes mild depression. That comes with being human, and is not necessarily depressing. Most marriages and love affairs and works of art and photograph albums are organised with a recognition of transience and limitation. To efface this sense, as Solomon seems to suggest, by kindly medicaments would be a human disaster, not an alleviation.

But the book soon gets to the hard stuff of severe depression. There is a harrowing account of Solomon's own breakdowns and the horrors of insomnia or excessive sleeping, almost total lack of energy, withdrawal, lost abilities to work or read, self-loathing, and - most awful - the feeling of near death. As the author points out, his has not been a particularly tragic life. His early feeling of sexual ambiguity and the death of his mother may have contributed to his illness, but in his detailed account of the chemical changes in the brain and body of those suffering from depression, he seems to point to some chemical malfunction in the brain. In his case, life has been made at least bearable by medication.

There is much in the book on medical treatments and on psychological counselling. Alternatives to mainline therapies are dealt with wisely and wittily ("the emperor has a whole new wardrobe in this business"). Among the alternatives described is a bizarre animist ritual called "ndeup" that Solomon travelled to Senegal to undergo. To the accompaniment of drums, the rite involves the rubbing of the body with millet and bathing in the blood of a sacrificed animal. This impressed him more than some group therapies practised in the United States.

Many things, however seemingly absurd, can help with depression, if they are believed in. The danger is that they may come to be seen as gods that fail, and plunge the depressive into even darker gulfs.

What is depression? Solomon says that each person's depression is different. What is the use of the word then, except as a clinician's catch-all? The older words more precise: anguish, despair, grief, terror, panic, anxiety, guilt. Depression may combine any or, most terribly, all of these. And they would all appear to be necessary in some way.

Without grief, we would not have love. We would abandon our dead where they lie and walk away without a care. These emotions are wired into our minds to be used. Perhaps the uses have changed so that in our modern society, they somehow fire at the wrong times and result in the confusion we call depression?

Then again, the symptoms are the same as Hippocrates described in the fifth century BC, that Durer showed in the famous engraving, "Melancolia", that Chaucer called "The Sin of Accidie". The same feelings Samuel Johnson and Winston Churchill named as "black dogs".

But somehow this illness seems far more widespread today. Solomon has fascinating sections on how different populations tend to have higher rates of depression: gays, in a largely heterosexual world; Jews in Gentile societies; blacks in white-dominated countries; women in a male-run world. The Inuit of Greenland have a disproportionately high rate: they endure long dark winters, and tend to live in quite large family groups in enclosed surroundings, where the only thing to do for the dark months is to "watch the walls melt". Perhaps hell is other people.

Andrew Solomon writes that he suffered a third breakdown while finishing this book. He left a message on his answering machine that he was "temporally unreachable". That seems as good a description as any for the wretched state of depression.

I hope that troubles are truly temporary, for he has given us will surely be a definitive study of depression, combined with a most moving personal narrative. Let his be the last words: "Every day, I choose, sometimes gamely sometimes against the moment's reason, to be alive. Is that not a rare joy?"