By Scott McLemme

PEOPLE WHO HAVE NEVER experienced depression, or witnessed its ravages up close, tend to think of it as a mood-a particularly keen variety of sadness, perhaps. This misunderstanding is forgivable. It is common to speak of "feeling depressed," after all. But for anyone who suffers from it, depression is not a feeling. It is a state of being. Depression is a world unto itself, one that is dense, with a lot of gravity, making it difficult to move, much less escape.

Or picture it this way. Imagine what goes on inside someone's mind just after a guillotine blade has done its work: absolute desperation combined with perfect helplessness, and the recognition that it's much too late to do anything anyway. Now take that millisecond of awareness and stretch it out-for a day, a week, a month. Longer. Depression means seeing only the portion of existence remaining visible from the basket that caught your head.

This is guesswork, of course, since I have never been beheaded; yet the metaphor does seem to fit chronic low-grade depression, a field in which my credentials are (alas) impeccable.

To convey the extremes of melancholia afflicting Andrew Solomon-which he writes about in "The Noonday Demon"-might require imagining something even more unnerving. Solomon describes periods when eating, sleeping and taking a shower represented major challenges. There were nearly insurmountable difficulties involved in moving from one side of the room to the next, much less picking up a telephone. It sounds like being at the very bottom of the basket, face down, with the weight of other lifeless flesh pressing on all sides.

That Solomon recovered to write about the experience for The New Yorker in 1998 was a tribute to antidepressants such as Prozac or Zoloft. And to that degree, perhaps, Solomon's memoir was also an advertisement for the psychopharmaceutical industry. These wonder drugs have become the first line of treatment for the disorder; in the age of managed care, they are often the last word as well, given their low cost compared to psychotherapy.

So part of the interest of "The Noonday Demon" comes from its status as evidence of medicine's continuing progress. Autobiographical passages recount Solomon's problem with a really harrowing psychiatric condition. Yet the book's sheer bulk (given its rambles through biomedical research and cultural history, and its digressive narratives based on interviews with other people suffering from depression) renders it oddly reassuring. The author is able to work hard; the problem hasn't clobbered him senseless.

Beyond this incidental encouragement, what Solomon provides, above all, is information. For someone who doesn't know much about the topic-but who wants (or needs) to learn a lot, as fast as possible-"Noonday Demon" will probably serve as the standard reference. It is a handy one-volume compendium of recent biomedical thinking about depression: the array of symptoms (from "low self-esteem" through suicidality), who is susceptible, what the brain-wiring problems are, how antidepressants work and what their side-effects can be.

While acknowledging the pop culture's fairly promiscuous use of the term "addiction," Solomon gives a substance-by-substance account of the various ways depressives try to self-medicate. His discussion reflects the sanest perspective on the link between depression and addiction: an understanding that they are closely linked and mutually reinforcing problems that, nonetheless, must be addressed separately. Treating one disorder while expecting this to cure the other sounds appealing, but the chemistry of the psyche is not quite so cost-effective.

Solomon also reports on the work of evolutionary psychologists who have argued that melancholia could have some long-term biological advantage: "Mild depression, in their view, causes intense introspection and self-examination, on the basis of which it is possible to make sophisticated decisions about how to effect changes in one's life so that it better suits one's character. ...A situation in which a person cannot disengage from a truly hopeless goal may be resolved through depression, which forces disengagement. ...Low mood serves to delimit persistence." It sure does. While the pages devoted to this argument are among the most intriguing in the book, the theory that melancholia offers its own rewards (in the form of profound cognitive enrichment) almost makes the whole experience sound appealing. Then again, this is an idea that can only be entertained when you have your wits about you-when you aren't trapped by the gridlock certainty that every single thing you have ever done (or can plausibly imagine ever trying) counts only as hopeless failure.

Much harder to take seriously is the speculation that depression could be nature's roundabout way of strengthening social bonds. It is nearly impossible to conduct a conversation when you have been decapitated. For one thing, you aren't a good listener; the scope for your attention is not far-ranging.

Solomon's commitment to making a comprehensive survey of the recent thinking obliges him to report on this theory. But as a sufferer, he knows better. "The fact is, most people are appalled by depression," as he rightly notes. "Though some respond to a display of depression with increased sympathy and altruism, more respond to such a display with revulsion and disgust." The comprehensiveness of "The Noonday Demon" is somewhat undercut by a tendency to repetition and meandering. Its large masses of information are held together, as by rubber bands, with anecdotal and confessional passages. For anyone with an unwelcome familiarity with its topic, Solomon's "atlas of depression" (as the subtitle has it) is something best taken off the shelf to consult, though not to read straight through. There are not many surprises here. But that's not a big problem. Sometimes you just aren't in the mood for surprises.