Ten years after her death, a new book of Mother Teresa's personal letters illustrates a profound and private spiritual struggle much of it unknown to the world that would come to embrace her as a living saint.

Mother Teresa: Come Be My Light, to be released Sept. 4, is a collection of Teresa's personal letters to her spiritual advisers. For the most part, they are letters she never intended to become public and, in fact, had asked to be destroyed.

In one letter from 1962, Teresa even mused about how her sense of spiritual desolation might impact the bidnow under way at the Vaticanto make her a saint.

"If I ever become a SaintI will surely be one of 'darkness,'" she wrote. "I will continually be absent from Heavento (light) the light of those in darkness on earth."

The book will likely challenge the characterization many people had of Teresa as a simple, pious woman, said the Rev. James Martin, a Jesuit priest who wrote the best-selling My Life With the Saints.

"I think that this is a real treasure for not only believers, but even doubters and skeptics," Martin said. "I think it also makes her much more accessible to the everyday believer. It shows that even the saints struggle in their spiritual lives and that they don't have it easier than we do. They sometimes have it harder than we do."

The book was edited by the Rev. Brian Kolodiejchuk, who directs the Mother Teresa Center from Tijuana, Mexico, and oversees her cause for sainthood; she was beatified by the late Pope John Paul II in 2003.

As part of the bid for sainthood, Kolodiejchuk read through 6,000 of Teresa's letters. For the book, he included letters pertaining to three aspects of her life: her vow to God, what she called "the inspiration" ...

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