It's likely that only Deborah Scroggins could imagine, from the lives of two very different but equally polarizing women, a fitting frame for understanding America, terrorism and Islam.

Scroggins' second book, "Wanted Women: Faith, Lies and the War on Terror" (Harper, 460 pages, $27.99), is a dual-track biography of Ayaan Hirsi Ali, the Somali refugee turned Dutch parliamentarian turned American neoconservative, and Aafia Siddiqui, an MIT-trained Pakistani neuroscientist whom the U.S. government calls the most senior woman in Al-Qaida.

Scroggins takes each as a symbol of "the defining conflict of our time" -- Ali the poster child of Western triumphalism, Siddiqui an emblem of jihad.

The dual-track biography is divided into thirds, with short chapters alternating between each woman's life. The structure is distracting at first, but it heightens suspense in the second and third sections, in which both women turn their versions of Islam -- and their personal ambitions -- loose in the world. Though their stories never intersect, they are united by each woman's rejection of moderate Islam for a fundamentalist interpretation: Siddiqui to enforce it; Ali to rail against it.

Siddiqui's own story is a true-life mystery: At one point, she disappears for five years. Theories abound: She is in hiding; she is a prisoner of the Americans, or the Pakistanis; she is being tortured, or she is dead. The Pakistani press stokes rumors, and when she finally turns up in Afghanistan, and ultimately in a New York court, she's become a national hero.

Meanwhile, Scroggins follows every lead she can, over six years and with seemingly little caution. At one point, a Pakistani double agent implicated in the murder of American journalist Daniel Pearl tries to wave her off the case; at another, a local fixer persuades her not to visit the family home of relatives of Khalid Sheikh Mohammed, the so-called 9/11 mastermind. (She still manages to get an interview with them, the only foreign journalist ever to do so.)

If Siddiqui's story is a mystery, Ali's is a soap opera. In this telling, the refugee becomes a diva, spending money she doesn't have, making outlandish demands for personal protection by the Dutch government, brazenly switching political teams when one doesn't advance her fast enough, breaking up a British power couple. Timothy Garton Ash once called her "an Enlightenment fundamentalist."

Ash was forced to recant the moniker, but by the end of Scroggins' book, it seems apt, even charitable. So fundamental has Ali become that she sells out her family, making her parents and half-siblings props in an argument about the vices of Islam, in not one but two memoirs. She visits her dying father only at the insistence of a Dutch ex-boyfriend; she speaks with her impoverished widowed mother only at the insistence of an American friend.

In Scroggins' book, Ali is more cunning than brilliant -- more Madame Bovary, say, than Voltaire.

Ultimately, Scroggins understands each woman as "an icon" whose "legend will always be more alluring than her reality." That's a powerful observation, subtly stated. "Wanted Women" is a necessary reminder that no individual's story is the best lens through which to understand this complex global story -- least of all when one casts oneself as the hero of that story.

Jina Moore is a writer who divides her time between New York and East Africa.