When hunting for reviews of a certain book, I recommend that you start narrow and then broaden your search if necessary.
Remember that not every book will necessarily get reviewed. Many reviews focus on books which create conversation--often because they are either very good or very bad.
Furthermore, not all reviews will necessarily be found online. If you don't find much online, locate print copies of journals from the time the book was published. Focus on key journals in the same area of history as the book (e.g., military history).