In it, Fr. Faber outlines the two views of Purgatory held by the Church, not contradictory views, and both valid. The first is that the experience of Purgatory is a bit like Hell, as punishment for our unforgiven venial sins is justly meted out. The second view is that the soul in Purgatory experiences the pain of knowing its need for purification before being embraced by the Beatific Vision.
The helplessness of the Souls in Purgatory is described, and Fr. Faber explains how prayers and Masses for the Holy Souls give honour to God's glory, and also allows us to exercise perfectly the theological virtues of Faith, Hope and Charity. Devotion to the Holy Souls also has a salutary effect on our spiritual life:
In the first place, it is a hidden work from first to last. We do not see the results, so that there is little food for vainglory; neither is it a devotion the exercise of which appears in any way before the eyes of others. It implies, moreover, an utter ignoring of self, by making away with our own satisfactions and indulgences and keeping up a tender interest in an object which does not directly concern ourselves... It leads us to think purely of souls, which is very difficult to do in this material world... We thus gain a habit of mind which is fatal to the spirit of the world and to the tyranny of human respect, while it goes far to counteract the poison of self-love.
On a slightly less serious note, I jokingly observed to Fr. Tim that the fires of Purgatory would have the advantage of being warm (I'm nearly always in jumpers and my car's air conditioning only gets used when other people are in my car.) He was quick to point out that, in my case, they were going to be something in the region of "arctic". Somehow I don't think he was joking...