In letter 13, Screwtape reprimands Wormwood for allowing his patient to repent and enjoy simple pleasures, such as reading a book or taking a walk for enjoyment. If the patient continues on this path, he will realize that simple pleasures are far better than the hollow pleasures of the world.
In letter 14 something alarming is happening. Screwtape says that Wormwood's subject is growing in humility, which is a key Christian virtue. In light of this, Screwtape urges Wormwood to get his patient to focus on his humility, so that he will be proud of his humility (paradoxically, I know). False humility is no humility at all.
Letter 15 is the most insightful. In this letter, there is a lull in the war. So, Screwtape urges Wormwood to make people think of the future. Screwtape argues, paradoxically, that the future is the least like eternity. The future is the place for fears and lusts. In this way, people cannot focus on the present and do what is best. Here is an excerpt:
Hence nearly all vices are rooted in the future. Gratitude looks to the past and love to the present; fear, avarice, lust, and ambition look ahead. Do not think lust an exception. When the present pleasure arrives, the sin (which alone interests us) is already over. The pleasure is just the part of the process which we regret and would exclude if we could do so without losing the sin; it is the part contributed by the Enemy, and therefore experienced in a Present. The sin, which is our contribution, looked forward.