Nineteen million Americans, says the National Institute of Mental Health, suffer from a condition that we in the industrial world have agreed to call chronic depression. This is a rubbery statistic: Legions more bear their grief without the counsel of a doctor, psychiatrist or social worker and are never diagnosed; others are treated instead for mysterious chronic disease in order to avoid the stigma of mental illness; some are judged to be “willfully noncompliant” and muddle through life on the fringes of the economy. And while “it is a mistake to confuse numbers with truth,” writes Andrew Solomon in his sweeping meditation on the illness, The Noonday Demon: An Atlas of Depression, “these figures tell an alarming story” that’s only getting worse. In 1980, only 1.5 percent of the population counted itself depressed. In two decades, that figure has more than tripled.

Do these escalating numbers merely represent an increased awareness about the disease, a sensitivity to the disabling nature of depression that results in more frequent diagnoses? And who sorts the treatment-worthy from the merely sad? How, in fact, do we define depression? The fourth edition of the Diagnostic and Statistical Manual (DSM-IV) — “psychiatry’s bible,” says Solomon — “ineptly defines depression as the presence of five or more on a list of nine symptoms.” The Comprehensive Textbook of Psychiatry proposes analyzing urine for evidence of neurotransmitter functions, as if the content of pee is a useful measure of brain chemistry. Solomon throws up his hands. “The only way to find out whether you’re depressed is to listen to and watch yourself, to feel your feelings and then think about them. If you feel bad without reason most of the time, you’re depressed.”

In nearly 600 pages of writing, all of it vivid with metaphor and thorough in detail, Solomon never draws any more indelible boundaries around this thing called depression. He offers up no sturdy test to assist the lay reader in making judgments about which suicides might have been prevented or whether a family member ought to be in psychiatric care. He contradicts himself often and admits the bias that comes with his heritage (his father worked at Forest Laboratories, a pharmaceutical company); he neither endorses completely nor rejects any etiology. Which, given his subject, is only right: As much as we want to analyze brain chemicals like blood sugar, or to split open the cerebellum and isolate the diseased area with a lighted pointer (“See! There it is! That’s why you feel so bad!”), such a finite understanding of abstract moods isn’t possible. Solomon’s refusal to come down on the side of any one treatment or theory is not a weakness; it is, instead, what makes his book definitive: By quoting everyone from terrorists to pharmaceutical experts and giving weight to every treatment from drugs to meditation, Solomon manages to give the reader a sense of depression’s shape-shifting, protean character. It is, perhaps, a truer picture than one could find in a scientific journal.

Depression has been blamed on the modern conditions of technology, stress, diet, genetics, faithlessness, and the abuse of certain drugs, but depression as an affliction is old enough for Hippocrates to have theorized, with astonishing prescience, about its physical causes (the word melancholy comes from the Greek for “black bile”). The early Roman Christians determined that melancholia was “a turning away from all that was holy” and regarded deep depression as “evidence of possession,” but didn’t deny its existence: The 90th Psalm of the Roman Catholic bible talks of a “noonday demon” that the fifth-century cleric Cassian interpreted as a reference to melancholy. Victorian England systematically set out to cheer up its depressives in asylums, and finally, in the early 20th century, treated them with drugs.

The rise of psychotropic treatments have, in fact, shaped our very thinking about mental illness. Peter Kramer writes in Listening to Prozac that “the success of lithium set off an explosion of precise psychiatric diagnosis”; in the same way, the advent of mood-enchancing pharmaceuticals such as monoamine oxidase inhibitors (which halt the breakdown of mood-determining neurotransmitters in the digestive system) and selective serotonin-reuptake inhibitors (which slow down the brain’s reabsorption of serotonin) has refined medical science’s understanding of depression. “As soon as we have a drug for violence,” Solomon says, “violence will be considered a disease.”

Solomon rejects the notion that mental illness arises simply from an imbalance in that notorious trio of neurotransmitters, norepinephrine, dopamine and serotonin. At the same time, it’s clear from the story of his own battle with depression that he would not have written this book — he might not have lived to write this book — had his depression not been treated by a thoughtful psychiatrist, with a customized cocktail of psychotropic remedies. And of all the information packed into The Noonday Demon, the particulars of Solomon’s own case history, which includes several breakdowns so severe he could not eat for days, make the most persuasive case for compassion. His depression hit “after I had pretty much solved my problems,” he writes, “when life was finally in order and all the excuses for despair had been used up.” His novel, A Stone Boat, had been published to good reviews, he had bought his first house. But after emerging from the hospital following an attack of kidney stones, his moods began deteriorating steadily. At its low ebb, his depression so debilitated him that he could not speak, cry or bring himself to turn over in bed, much less get out of it. “The task,” he writes, “seemed colossal.”

The Noonday Demon has its beginnings in an article Solomon wrote for The New Yorker, “Anatomy of Melancholy,” in the aftermath of his first breakdown; Solomon subsequently expanded it in the structure of an atlas, spread over 12 chapters covering history, evolution, treatment and suicide. He unearths insanity in nearly every stratum of society, interviews scores of depressives, investigates history for evidence of depression and its cures. Poverty may be an indicator of depression, as is drug addiction, violence, anger and even, in the case of Emily Dickinson, isolation from society and a compulsion to write poetry. Depression to Solomon is like holiness to a Buddhist: He finds it in everything.

Unlike Kay Redfield Jamison, who in her memoir An Unquiet Mind depicted herself in a swirl of adoring men and academic achievement, mitigating the stigma of her manic-depressive illness by demonstrating that the attractive, smart and rich can also be crazy, Solomon attacks stigma by telling his story with an almost embarrassing candor and very little drama. He plunges headlong into his darkest shadows. “I will never forget the feeling of his face crumbling under my blows,” he writes of an incident following a “falling out” with a friend he believed had betrayed him. The friend forgave him. By the time Solomon gets to his treatise on depression and poverty, in which he argues persuasively for a social policy that locates and treats depression in the indigent, he has set the stage for compassion. “In this era of welfare reform, we are asking that the poor pull themselves up by their bootstraps,” he writes, “but the indigent who suffer from major depression have no bootstraps and cannot pull themselves up. Once they have become symptomatic, neither reeducation programs nor civic citizenship initiatives can help them. What they require is psychiatric intervention with medication and with therapy.” Two-thirds of the way through The Noonday Demon, Solomon does not have to convince his readers that depression is real; by the force of his own narrative, we know. No doubt it affects the poor in equal numbers.

“Sometimes I wish I could see my brain,” Solomon writes. “I’d like to know what marks have been carved in it. I imagine it grey, damp, elaborate. I think of it sitting in my head, and sometimes I feel as if there’s me, who is living this life, and this strange thing stuck in my head that sometimes works and sometimes doesn’t.

“It’s very odd,” he concludes. “This is me. This is my brain.” The alchemy of both, however it worked, produced an impressive book.

THE NOONDAY DEMON: An Atlas of Depression By ANDREW SOLOMON Scribner | 571 pages | $28