Don't do a chapter-by-chapter summary of the book; instead, focus on the overarching goals and themes and draw on specific chapters as illustrations. Elaborate on and contextualize the significance of the project, be fair, thoughtful, and diplomatic in articulating your critiques, and wrap up with forward-looking praise. And (speaking as a one-time book review editorial assistant) get it in on time and slightly under the word limit.
This is what my gut was telling me.
Thanks for all the help, people!