ZAWAHIRI AND BIN LADEN Whereabouts unknown.

Exactly Five years on, we're still looking for explanations, meanings, answers. Why did they do it? How could this happen?

So, when New Yorker staff writer Lawrence Wright published his 400-page primer on al Qaeda last month (and with such a sweeping and promising title), I was hopeful that definitive answers were on the way. (Wright's magazine articles about al Qaeda had been the richest Cliffs Notes anthropologies on radical Islam written in the wake of 9/11.)

Wright conducted an astounding number of original interviews for this book—including this coup: an interview with one of Osama bin Laden's teenage daughter's playmates from the late '90s when the bin Ladens lived their back-to-the-land existence in the terrorist compounds of Afghanistan. (Turns out bin Laden doesn't object to his daughter's music cassettes. "He's not really that hard," she tells her friend; "he just acts like that in front of the men.")

Wright also delivers an encyclopedic primer on the developments and events (both quiet and notorious) that led up September 11, 2001: Sayyid Qutb's martyrdom; the fanatical attack on the Grand Mosque in Mecca in 1979; Anwar Sadat's assassination in 1981; bin Laden's emergence during the Soviet war in Afghanistan; the birth of al Qaeda (Wright has the minutes from the first al Qaeda meeting in 1988); the fatal disconnect between the CIA and the FBI. And he details the infamous news clips from the 1990s—the first Twin Towers bombing, Black Hawk Down, the coming of the Taliban, the embassy bombings in Kenya and Tanzania, and the deadly ambush on the USS Cole in 2000.

From the conspiratorial jail cells of Cairo, to the FBI's linchpin interrogation of an al Qaeda operative in Yemen, to the late-night soirees among the green corduroy settees in the salons of Khartoum, Wright also gets that there's a Graham Greene novel in all this, and he colors his history lecture with the shadows of a sultry thriller.

Indeed, history becomes hard-boiled literature in scenes like the one Wright draws in murky Afghanistan when Taliban leader Mullah Omar receives the leader of Saudi intelligence, Prince Turki bin Faisal al-Saud, in a decrepit Kandahar guesthouse. Omar disappears for 20 minutes and then, upon his return, using his translator as cover, tells outright lies to cancel a deal to hand over bin Laden. "Turki began to wonder if [Omar] was on drugs... Turki and General Rana rode back to the airport in stunned silence. It was galling to pass by Tarnak Farms, bin Laden's dilapidated citadel. From now on... Saudi Arabia's place in the world would be held hostage by the man inside."

Wright does provide answers to the mystery story. But he doesn't do it through pronouncements or analysis. The answers come in intimate descriptions of bin Laden and Ayman al-Zawahiri, al Qaeda's infamous leaders. Wright humanizes these two cartoon bogeymen. It's by no means a sympathetic treatment (Zawahiri in particular comes across as a carnivorous psychopath whose Orwellian manhandling of Islam provides the justification for suicide bombings). But in giving both men narratives that are substantiated with mundane human drama—bin Laden's off-key efforts in the Sudan to emulate his dad's success as a master builder, Zawahiri's petty rivalry of machismo with Egyptian radical Sheikh Omar Abdel Rahman—they become real people. (Bin Laden, by the way, is 6' tall, not 6' 5", Wright reports.) Rendering Zawahiri and bin Laden as human beings doesn't give us specific answers, but it does place the story in the real world—where motives steeped in human emotions and grudges and dreams are easier for us to comprehend.

Then again, even bin Laden's comrades are left puzzled. When Islamic FBI agent Ali Soufan cracks the case—Soufan is my favorite surprise in the book—and proves al Qaeda is behind the attacks, Wright writes: "He took seven photos out and laid them on the table. 'How do you know?' asked [al Qaeda operative] Abu Jandal. 'Who told you?' 'You did,' said Soufan. 'These are the hijackers. You just identified them.' Jandal blanched. He covered his face. 'Give me a moment,' he pleaded. Soufan walked out of the room. When he came back he asked Jandal what he thought now. 'I think [bin Laden] went crazy,' he said."

Dazed, Jandal begins to talk.