Study the title of the book. Is the title catchy and interesting? Comment on how the title ties into the book's text.
Identify the genre of the book. Does the text of the book fit the intended genre? If you can't figure out what the genre is then most likely the reader won't be able to either.
Comment on the length of the book. For instance, a novel should be 80,000 to 120,000 words. Does this book follow those guidelines? If the book is longer than the maximum, then the author of the book might consider breaking the novel into parts or doing a careful edit to remove excess. If the novel is shorter than the suggested minimum, then the author may need to go back and add more detail.
Read the opening lines of the book. Comment on whether or not you found the opening lines engaging enough to hold your interest and make you want to keep turning the pages. A good novel quite frequently opens in media res (in the middle of the action).
Analyze the dialogue. The dialogue should follow normal speech patterns and not sound forced or clipped. On the other hand, authors should beware of using too much straight dialect or they will lose their readers.
Discuss the overall plot and theme of the book. What is the book trying to say and is it said effectively? Does the book have patterns that are carried out throughout? Comment on the use of symbolism, metaphor, and tone. Are the characters developed enough and do they contribute to the plot? You get the idea.
Check the mechanics. Finally, comment on the overall mechanics of the book--the grammar, spelling, and punctuation.