If you’ve read a book thoroughly and want to say “I don’t understand”, then you are saying the book has a fault. Your job is to locate the source of the trouble. Perhaps the book’s structure is poor. Or it’s parts don’t work well together. Or it rambles and parts are irrelevant. And so on.
Acknowledge all the emotions you bring to the table. Make them explicit. Otherwise you will likely vent feelings, and not reasons.
You must also make your assumptions explicit. Know your prejudices. And realize your opponent is entitled to a different set of assumptions.
Good controversy should not be a quarrel about assumptions.
If you have not been able to read a book sympathetically, your disagreement with it is probably more contentious than civil.
If you understand but disagree, you can make the following statements to the author:
- You are uninformed
- You are misinformed
- You are illogical. Your reasoning is not cogent.
- Your analysis is incomplete.
This list is not exhaustive, but likely most conclusions will be one of these four.
Do not quibble over lack of knowledge that would not change the conclusion. Nor about errors that would not change the conclusion.
If you think the analysis is incomplete, perhaps he did not solve all problems he stated? Or he did not make adequate use of his materials. In any case, whatever the reason, justify why you say it.