Quality is much more important than quantity.
Your book review should show evidence, where appropriate, of your familiarity with the content of the required readings and the lectures. I distinguish between book reviews and book reports. A book report is almost wholly descriptive and usually is little more than an expanded table of contents interspersed with a few paraphrases from the book jacket blurb. Book reports are not acceptable for this course. A book review is a critical evaluation of the content of a book, based on the reviewer's familiarity with the subject.