1. Before you form your own opinions, try to understand what the author intended to do -- to the extent that you can figure it out. Identify the major aims and sub-goals.
To accomplish this you should pay careful attention to prefaces, introductions, and conclusions -- as well as to other kinds of help the author offers you, such as chapter and section headings. You may want to identify the intended audience at this point.
2. After you have identified the author's goals, you should assess the extent to which those goals were achieved.
Do not at this point complain that you would have had other goals or that for some reason the author should have tried to do something that he or she did not try to do.
3. If, after you have identified the author's aims and evaluated the extent to which those aims were achieved, you don't "buy" the text, you may offer suggestions based on what you would have liked to have seen done with the topic.
It may be useful to comment on the author's style and on the appropriateness of her/his vocabulary to the intended audience. It is, however, not valuable to the reader to be told simply that the whole topic is boring and that one's time would be better spent watching the grass grow and the cows eat it -- or to criticize the author for using a specialized jargon if she was writing for a specialized audience.
4. You should comment on special features of the work, for example on illustrations
if they are unusually good or bad, on appendices if they are important or useful,
on such helps as glossaries, time lines, annotated bibliographies, or short
Here again, you want to engage with the intended purpose and the intended audience of the text before you criticize its illustrations.
Always follow these guidelines when you read a text; as a matter of course, they apply as well when you make a presentation about a text.