In letter 18, what does Screwtape describe as both “the joke” and “the truth”? What does he mean by this?

The sentence that uses a demonstrative pronoun is the sentence: That was my book and you took it. The demonstrative pronoun used here is "that". Demonstrative pronouns are used to indicate something specific in the sentence. These demonstrative pronouns include these, that, this, and those. Demonstrative pronouns usually identify nouns.