Letters 18-20 are about love. If Wormwood can misdirect his patient in his understanding of love, then the patient will go awry. Two points stand out. First, since all people are fallen, their love is also selfish. So, if Wormwood can work on that point and make the patient love himself in his pursuit of love, then Wormwood would have done his job well. Second, Screwtape reminds Wormwood that shallow concepts of beauty are a great way to cause a person to marry the wrong person for the wrong reasons.
In letters 22-23 Screwtape is angry, because the patient is now dating a Christian woman. So, the tactics must change. Screwtape wants to attack the patient in a different way: the desire for novelty (letter 25) and bitterness in view of the perceived selfishness of other people (letter 26).
The last letters in your list concern the bombings that are about to take place in the city. With these bombings, Screwtape wants to cause temptation in a different way: prideful courage, shameful cowardice, or hatred of the Germans.
As you can see, these letters are attacks on Christians to prohibit them from growing in faith.
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