At a moment when the American public is dissatisfied with President Bush’s war on terrorism but lacks any sense of what might replace it, it is useful to have another book on the men who destroyed the World Trade Center and the feeble effort to stop them. In “The Looming Tower: Al-Qaeda and the Road to 9/11” (Alfred A. Knopf, 480 pages, $27.95), Lawrence Wright, a New Yorker magazine writer who has produced books on topics as diverse as twins and criminal cases, has done an incredible amount of work, cornering an amazing array of sources, including childhood friends, teachers, and relatives of the men behind the attacks of September 11, 2001, and their American trackers.

The book’s drama is the competition of the two sides — extremist Muslim and American — who are themselves composed of organizations that sometimes cooperate, sometimes compete. As Al Qaeda prepares the attacks of September 11 and the American bureaucracies struggle to follow clues, the narrative picks up speed and drama. On the American side, the featured organizations are the FBI and the CIA, which seems to emerge as the American villain.On the extremist Muslim side are Al Qaeda and Al Jihad, Ayman al-Zawahiri’s organization, which eventually merge.

Mr. Wright, however, tells all of this as a “human interest” story of individuals. Among the Americans is the head of the FBI’s counterterrorism section from 1995 to 2001, John O’Neill. O’Neill resigned to take charge of the World Trade Center’s security shortly before September 11th and died that day, giving Mr. Wright’s drama its denouement. Mr. O’Neill’s earthy origin in lower-class Atlantic City, his flamboyant style and messed-up life — his mistresses kept ignorant of each other, violations of FBI rules, and lurches from religious experimentation to frenzied Catholic piety — make the narrative more interesting.

But O’Neill was simply not as important in the events as Osama bin Laden and Mr. Zawahiri were. Above all, it is hard for Mr. Wright to depict him as a hero because he shared the fundamental blindness of the FBI: the unquestioned belief that vast international conspiracies could be approached by putting a few individuals on trial and in jail. When Richard Clarke, President Clinton’s counterterrorism coordinator at the National Security Council, wanted to kill Mr. bin Laden, “O’Neill didn’t want to hear about it.” Mr. Wright writes. “He was a lawman, not an assassin.” Astonishingly, Mr. Wright never questions the adequacy of criminal investigation, indictment, and trial as an answer to Islamist terrorism.

Messrs. bin Laden and Zawahiri are ultimately much more interesting, and Mr. Wright’s presentation of them far less conventional. In Mr. Wright’s vast reportage on Mr. bin Laden and his relatives, wives, and children, he emerges as a leader with major limitations. “Academically undistinguished himself, and clearly uninterested, bin Laden would never pursue the respectable professions.” A friend during his years in Sudan who “loved that man” also remarked, “Unfortunately, his IQ was not that great.” He was a good fund-raiser, but “a terrible failure” as a businessman in Sudan. “‘He had a small smile on his face and soft hands,’ a hardened Pakistani Mujahid recalled. “You’d think you were shaking hands with a girl.'” Frequently unwell, Mr. bin Laden was long afraid of combat in Afghanistan, and his first military operations were ridiculous failures. So it comes as a surprise when Mr. Wright concludes that September 11th would not have happened without Mr. bin Laden’s “vision … leadership … tenacity … artistry.” His presentation of Mr. bin Laden is fascinating, but never quite jells.

Mr. Wright’s book has its greatest value in presenting Mr. Zawahiri, officially Mr. bin Laden’s deputy in Al Qaeda, as ultimately the more important figure. Mr. Wright says “the story of al-Qaeda had really begun in America,” with the 1948–49 visit of the ideologist of today’s Islamist extremism, Sayyid Qutb, the young Mr. Zawahiri’s inspiration and his uncle’s teacher. In 1966, the teenage Mr. Zawahiri founded a clandestine organization, Al Jihad, devoted to overthrowing the secular Egyptian government. Al Jihad assassinated Anwar Sadat in 1981, and Mr. Zawahiri was tortured, forced to inform, and convicted. Both Messrs. bin Laden and Zawahiri were subsequently caught up in the CIA-organized campaign against the Soviet occupation of Afghanistan.

Each man filled a need in the other. Zawahiri wanted money and contacts, which bin Laden had in abundance. Bin Laden, an idealist given to causes, sought direction; Zawahiri, a seasoned propagandist, supplied it. … [T]he organization they would create, al-Qaeda, would be a vector of these two forces, one Egyptian and one Saudi.

Until he met Mr. Zawahiri, Mr. bin Laden “had never voiced opposition to his own government or to other repressive Arab regimes.” According to Mr. Wright, Mr. Zawahiri taught Mr. bin Laden the heresy of takfir, the view that unfriendly Muslim leaders could be declared infidels. During the Afghanistan war, one eyewitness noticed “how the Egyptians formed a barrier around the curiously passive bin Laden, who rarely ventured an opinion of his own.” Mr. Wright implies that Mr. Zawahiri murdered Sheik Abdullah Azzam, the Palestinian Arab Islamist who figures as a sort of hero on the Islamist side. In the early 1990s, Mr. Zawahiri put his Al Jihad organization on the payroll of Al Qaeda “as the only solution to keeping the Jihad organization abroad alive.”

After Mr. bin Laden went to Sudan, Mr. Zawahiri pioneered his relationship with Iran and its Hezbollah clients. It was Mr. Zawahiri, according to Mr. Wright, who introduced the suicide tactics, never before used by Sunnis, that were to become the key to Al Qaeda operations, and who developed “the theoretical framework” to justify killing the innocent. In Sudan, Mr. Zawahiri arranged for Mr. bin Laden’s bodyguards to be Egyptian, “drawing ever tighter the noose of influence he was casting around the Saudi.”When, in June 2001,Al Jihad and Al Qaeda finally merged, two-thirds of the latter’s leadership council consisted of Egyptians.

In arguing the importance of Mr. Zawahiri, Mr. Wright makes a strong case. Mr. Zawahiri is a cosmopolitan doctor who was the product of two famous families, one religious and professional, the other political, in Egypt, the center of Arab culture. He contrasts with Mr. bin Laden, the uneducated son of a rich parvenu in one of the least sophisticated Muslim countries. Mr. Zawahiri was far more familiar with terrorism from Egypt, which has a long tradition of anti-Western conspiracy, assassination, and bombing.

“The Looming Tower” ends with Mr. Zawahiri escaping into Pakistan after the American invasion of Afghanistan. Mr. Wright’s emphasis on Mr. Zawahiri over Mr. bin Laden testifies to the author’s open-mindedness. Unfortunately, he is hampered by a fundamental lack of knowledge of Islam. He gets many things right, such as the importance of Wahhabi or Salafi doctrine and of Qutb. But he makes many errors. To say Islam requires the rule of clerics is precisely the modern innovation of Ayatollah Khomeini and the Taliban, the result of the transmutation of religion from a guide to life into a political ideology modeled on communism and fascism.

Unfortunately, Mr. Wright also lacks the awareness of how politics can be twisted by ideas. He reports the Islamists’ use of the Leninist words “vanguard” and “front” without ever noting how distant these terms are from traditional Islam. He quotes, but passes by, an American Marxist of the 1960s who converted to Islam and told Mr. Zawahiri,”When I talk to you I feel like I’m back in the Party. I don’t feel as if I’m with a traditional Muslim.” For Mr. Wright, one feels, the 1960s, Chairman Mao, and his Western admirers are already as distant as the age of Muhammad.

If this feeling is a harbinger of our future, it will become even harder to deal with Mr. bin Laden, Mr. Zawahiri, and their future imitators. September 11 is unimaginable without ideas like “vanguard” and “front” (What were the international Saudi charities but front organizations?) as well as jihad, takfir, and shuhada, or martyrdom.

Mr. Fairbanks is a former deputy assistant secretary of state. He is currently at the Paul H. Nitze School of Advanced International Studies at Johns Hopkins University.