One book should address your theme in the region or country on which you specialize, the other should explore that theme in another geographical context. Ina review you should provide brief summaries of the works under review, but more importantly, you should make clear to your readers the significance of the issues under consideration by the books and the key issues on which they agree and disagree, and you should render your assessment of the contributions of the works to our understanding of the issues they explore. You are not reviewing these books for stylistic devices, but for the substantive contributions they make to our knowledge and understanding of key issues. Perhaps the best way to learn what a critical review should do is to read some examples.
Critical reviews differ depending on the audience for which they are intended.
Some critical reviews are written to the intellectual community broadly conceived. Examples of these can be found in journals of criticism such as The New York Review of Books and The New Republic. Others are written to a more specialized scholarly audience.