Samantha Geimer, the girl in the Roman Polanski rape case, has published what might be the most important and valuable book of the century so far.

It may prove to be one of those books that a lot of people talk about without actually reading, like A Brief History of Time, or The Tipping Point, or most of the school syllabus.

But that's OK. The value of Geimer's book, The Girl, lies in the debate it stirs up; this is already happening through serialisation and widespread, articulate interviews with the author. If that triggers a bigger discussion among non-readers, then she has still done something useful and important.

How much do you know about the story? I knew a bit, but still experienced what hurried book reviewers call "an emotional rollercoaster" while reading one of her interviews.

When Samantha Geimer was 13, the famous 43-year-old film director Roman Polanski said that he was photographing young American girls for a feature in French Vogue. With forgivable naivety, Geimer's mother allowed him to take her out alone. He photographed her topless, which she did not tell her mother.

A few weeks later, Polanski took Samantha to Jack Nicholson's house, gave her several glasses of champagne and part of a sleeping pill, then had sex with her. It was statutory rape. Geimer says: "It was rape in every sense of the word. I said no."

The way in which he had sex with her is indelicate to include, but important. Geimer's book expresses it with literate sarcasm: referring to a sympathetic psychological report after Polanski's arrest, which cited his "solicitude concerning pregnancy" as a mitigating factor, Geimer says this was "an interesting new euphemism for sodomy".

I don't know how this makes you feel. It fills me with thoughts of violence. I imagine being alone with Polanski, kicking and punching him. The anger I feel, at the thought of this being done to a drugged child, seems to be an instinctively brutal one.

Then you read about the life of Roman Polanski. How shameful and how pointless to punish him with violence, even in the imagination.

Aged six, he saw his father taken to a concentration camp. His mother died at Auschwitz when she was four months pregnant. At 35, with God knows what ineradicable scars, Polanski married Sharon Tate and they started a family immediately. Tate was eight months pregnant when a gang broke into their home, stabbed her to death and smeared "pig" on the front door in her blood.

This is not an excuse; other survivors have not become rapists. But it silences my violent instinct immediately and creates a sharp and terrible sympathy in parallel with the anger. A second complicating factor is that Polanski's work is filled with beauty and humanity.

These are unfamiliar feelings; our modern world does not invite us to treat anybody as nuanced. People are heroes or villains, victims or victimisers; sometimes neither, but never both.

When Roman Polanski, who has lived in exile from America and its justice system for decades, was nominated for an Oscar for directing The Pianist, Samantha Geimer called on the Academy to "judge the movie, not the man".

She has been exchanging emails with Polanski for several years.

She says that the police investigation, hospital exams and reporting of the case were more traumatic than the attack itself. She says: "I did something wrong, I was stupid… To pose topless, and to drink and to take the [sleeping] pill."

It is so easy and tempting to knock this into a pigeonhole: the misguided self-blame and denial of the victim. But this woman is too smart and articulate for us comfortably to assume we know better. She puts these complicated thoughts out there, alongside her anger, not because she's too damaged to think clearly but because she can't bear the world's oversimplification.

When a therapist on the Oprah Winfrey show explained that Geimer was suffering from "victim's guilt", she said this was "patronising"; who would dare patronise her further by saying that it wasn't?

In The Pianist, Polanski transformed his ghastly knowledge of the camps into an act of artistic self-expression. In The Girl, Geimer does the same with her rape. That is a powerful response, from both of them. But what an incredibly complicated common bond.

It is the complication that we need. People have become desperate to reduce everything, including each other, to mindless categories of good and bad, as if the world can be divided into Facebook likes and dislikes.

When I wrote about the Muslim women in Birmingham who were protesting against a ban on the niqab, and the argument that they are so deeply in the patriarchal grip that they cannot choose as freely as they think, I pointed out that people have said the same to me about taking my husband's name. Many readers asked why I was defending the veil. Others pointed out the differences between veiling your face and changing your name. It was as though there is no room for analogy unless it's a direct comparison and no room for words on the niqab other than "Hurray for it" or "Ban it".

Similarly, we yearn to know if we should be cheering or booing at Operation Yewtree, political leaders or the idea of bombing Syria.

So what is to be done with Samantha Geimer's story? She does not condemn Polanski nor exonerate him. She does not blame herself nor refuse to examine herself. Her voice is strong and complicated. You cannot simplify her, or him.

Her current battle is not with her original oppressor but the reporters of then and now, the lawyers, the psychologists of reality TV and everyone watching – all of whom objectified her further. She is fighting against reductive simplicity. She forces us to think hard, to use muscles that must not go slack.

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