The Latin American Book Store, Ltd., a vendor of books from Latin America, has complied on their website a list of "selected web resources that compliment the discussion of Latin American academic books." It is well worth checking out!
MLA International Bibliography (MLA), JSTOR, and ProjectMuse can all be extremely useful for finding scholarly book reviews, as can the interdisciplinary database Academic Search Complete. These databases cover, in addition to book reviews, scholarly journal articles, dissertations, editorials, and speeches, so you will likely wish to limit your results to book reviews. Unfortunately, no designated book review field exists in MLA, but the phrase "review article" is often used in a record's subject field for this purpose. It is easy to limit your search to "Reviews" in JSTOR.
The following is a dedicated book review databases: