What initially drew me to the book was the idea of faith in the face of adversity--Mother Teresa is the posterchild of a "good" Catholic and since her death in 1997 has been beatified, the first step towards becoming canonized as a saint. Despite her "saintliness," however, she struggled with doubts, mired in what some call the Passion of Christ--that moment on the cross when Christ cried, "My God, My God, why have you forsaken me?" She felt abandoned by God, undeserving and unloved, and she still managed to live her entire life for the poor, the sick, and the dying. That kind of faith is staggering to the average American agnostic (a.k.a. me).
The book has been hailed by the religious community as an example of true faith to bolster the masses who also might have moments of doubt:
"[Rev. James] Martin of [the Jesuit magazine] America, a much more liberal institution, calls the book 'a new ministry for Mother Teresa, a written ministry of her interior life,' and says, 'It may be remembered as just as important as her ministry to the poor. It would be a ministry to people who had experienced some doubt, some absence of God in their lives. And you know who that is? Everybody. Atheists, doubters, seekers, believers, everyone.'" 
The letters themselves are beautiful and fascinating. Though she was Albanian by birth, most of the later letters are written in English and have an almost-poetic feel to them, emphasized by her habitual use of a dash as punctuation which often serves to create a rhythmic flow. Read this excerpt and tell me it's not lush:
Lord, my God, who am I that You should forsake me? The Child of your Love — and now become as the most hated one — the one — You have thrown away as unwanted — unloved. I call, I cling, I want — and there is no One to answer — no One on Whom I can cling — no, No One. — Alone ... Where is my Faith — even deep down right in there is nothing, but emptiness & darkness — My God — how painful is this unknown pain — I have no Faith — I dare not utter the words & thoughts that crowd in my heart — & make me suffer untold agony.
One: How are the author’s psychological conflicts revealed in his or her work?
Two: What is an in-depth analysis of the characters if they were real people?
Three: What is the appeal of the work to the readers in relation to their own ability to work out hidden desires and fears? 
 Van Biema, David. "Mother Teresa's Crisis of Faith." Time. 23 August 2007. http://www.time.com/time/world/article/0,8599,1655415-1,00.html
 A Dictionary of Literary and Thematic Terms. Ed. Edward Quinn. Facts on File, Inc. New York: 1999, page 263. Accessed at http://www.northern.edu/benkertl/psychological_criticism.html