The before and the after are stark. Before, you don’t think suicide has anything to do with you. After, you know it does — undeniably, irreversibly. The membrane separating those two states is whisper-thin. Whether you cross it is not at all up to you.

Joan Wickersham entered the after on a February morning in 1991, when her 61-year-old father, Paul, got dressed, retrieved the newspaper from the end of the driveway, made his wife a pot of coffee, and brought a cup upstairs to her while she slept. Then he went into his study, shut the door behind him, sat down in an armchair, put a Colt .38-caliber revolver in his mouth, and blew a hole in his brain. Probably he had no idea that he was blowing a hole in his family as well.

“Did he know what it would do to us — my mother, my sister, and me?” Ms. Wickersham writes in her memoir, “The Suicide Index: Putting My Father’s Death in Order” (Harcourt, 316 pages, $25).

“If so, then he did something unforgivable.

“If not, then I wish he had known. But only if he really did have a choice, and only if knowing would have stopped him.”

I wish I could know what it is like to read “The Suicide Index” from a distance, from the seeming security of before. But my family was ripped from that illusion of safety five years ago next week, when my uncle — my brilliant, funny, kind, tormented uncle, my favorite, my brothers’ favorite — shot himself in the head in his suburban backyard. And so I read “The Suicide Index” with a rapacity bordering on need, with tears in my chest and in my eyes. Occasionally I had to put it down and leave the room. More often, I devoured it.

The book is the product of a loving daughter’s grief, and part of her process of grieving. But it is also the measured, elegant, gripping work of a professional writer who has set her powers of observation to work on her own family — her parents and grandparents, her uncle, her sister, her husband, her son — and on herself. As the title suggests, it is structured not by chapter or even straight chronology but by entries in an index, starting with “Suicide: act of, attempt to imagine.” “The Suicide Index” itself is largely an attempt to imagine.

In the aftermath of a suicide, “Why?” is an inescapable question; it is the question that not only survivors but strangers ask when they hear of a self-inflicted death. Sometimes the answer is obvious. Not so with Paul’s death. The possible causes are both myriad and, to his daughter’s mind, insufficient, too surmountable to leave death as the only option: financial troubles, deteriorating health, the violent abuse he suffered as a child at his father’s hands, the blossoming friendship between his wife and another man.

To turn back the clock and stop her father is what Ms. Wickersham wants. Failing that, she wants at least to understand — to understand not only why her father killed himself but who he was, this gentle, reserved, secretive man who left no note and, while he was alive, showed no discernible signs of planning his own exit.

“Biography, in the case of someone who commits suicide, is particularly dangerous, misleading,” Ms. Wickersham writes, ever cautious, ever self-aware. “It looks at a life through the lens of a death. Every time a bad thing happens, the temptation is to say, ‘Aha!'”

That’s the cultural tendency as well. Suicide is a cause of death that is like no other in its ability to eclipse the details of a life. A person who dies of cancer or old age, who is killed in a car accident or a war, is not defined by that death. But as soon as someone kills himself, his life is reduced to the cause of death, as if the dangerous instability that led to his end is the sole fact anyone needs to know about him. All else is written off. “It’s the only cause of death that can be used as a noun to describe the dead person,” Ms. Wickersham points out. “Your death becomes your definition.”

Yet this compassionate, loyal, quietly keening daughter will not allow that to happen to her father, not even in a book about his suicide. Yes, she examines his life through the lens of its end, and because of its end, but the portrait that emerges is not merely a death mask. Instead, she renders a fully three-dimensional human being, albeit one permanently unavailable to answer the question that haunts her even now.

She also shows us three generations of a family in mourning, learning how to navigate the long shadow of her father’s death. The hole her father blew in the family is there still, all these years later: crippling, yes, but not fatal.