It's a good question. First, the books are always similar in topic. (Piper's book about what Jesus demands from the world is a constant in Barham's two-book rotation, but he tends to sub in others as his second.) He starts with a sentence in one book and reads right across to the sentence in the other book. Lately, he's been throwing in a third book that he studies more than reads. He insists that he retains it all.