MENTAL ILLNESS IS A DIFFICULT subject for me. Last week, I watched the movie Young Adult and almost threw up at the appallingly light-hearted treatment of the main character, who is unquestionably and seriously ill. Mental illness runs in my family, and in me, and so I tend to stay away from personal narratives in the vein of The Guardians. But Sarah Manguso needs to tell us something, and we need to listen. She needs to tell us about her friend Harris who jumped in front of a train.

Here are the facts: Sarah and Harris met in college, living together among a rotating crew of young people in a New York City loft. Together, on 9/11, they watched the towers fall from the bank of the East River, then rode out to Harris’s parents house on Long Island and ate candy. Harris was a gifted musician. In his life he had a total of three psychotic breaks for which he was institutionalized. During the third, while Manguso was living in Italy on a writing fellowship, Harris somehow walked out of the hospital, went missing for ten hours, and then jumped in front of a train.

The Guardians is a requiem, written by a friend in the beginning stages of the messy, lifelong process of grief. “I could have waited until the end of my life to try to understand what happened on that day,” Manguso writes, “saved it for last so I could know its whole effect, but instead I waited what seems an arbitrary, meaningless length of time.”

What makes this book so revelatory, what sets it apart from the seemingly endless succession of recent grief memoirs, is that it all seems to happen in real time. The Guardians is like a Polaroid, developing while you watch; Manguso works through her despair moment by moment, on the page, right in front of you. Because she’s experiencing and documenting at the same time, she has no clear perspective. And yet it turns out this might be the best way to tackle the subject. After all, what perspective does one ever get on suicide? At most, you gain some muted sense of acceptance, which is both negating and boring to read about.

So how does Manguso achieve this sense of collaboration with the reader? How does she make it feel like the words are appearing on the page you’re holding as she’s writing them? First, through a seeming disorder. The prose is fractured, ordinary, oral, random. The writer addresses herself, Harris, and us. Just when she’s in the middle of an anecdote about her friend, or providing some background information on the science of antipsychotic drugs — at precisely the moment you expect an assertion of authority — she drops in to remind the reader: she has no idea what she’s doing. She tells us, “Everyone writes, ‘What are you working on?’ I’m working on a book about a man who jumps in front of a train. I have no interest in hanging a true story on an artificial scaffolding of plot, but what is the true story? My friend died — that isn’t a story.”

She’s right. It isn’t a story. The Guardians is more of a rendering, in the literal sense — one where the author offers the conceit upfront: “I want to set aside every expectation of how I should feel or act given that my friend had a bad death, and try to explain what has actually happened to me — if, in fact, anything has actually happened to me.”

What happens when there are two people, and then one of them dies? What do the ...