I'd test one chapter (or one sub-chapter) at a time. Reader-reported assessments of quality and ease of understanding would be the obvious thing to measure for any instructional book... but also, isn't CFAR trying to come up with various "rationality tests"? Accuracy of responses to test questions would be a great metric to try and optimize, and most instructional material benefits from having end-of-chapter questions anyway if only to help readers verify that they've gained actual understanding and not just an illusion thereof.