Use JSTOR to find reviews of books with an imprint/copyright more than a few years old, and ProjectMUSE to find reviews of books published in the last several years. Please keep in mind that all three of these databases index substantive journal articles in addition to book reviews, so be sure to limit your search to "Reviews" in JSTOR, "Reviews Only" in ProjectMUSE, and "Book Review" in Academic Search Complete.
Information from the New Yorks Times Books Section
Some book review sources are not appropriate for scholarly work. Avoid reviews in popular magazines such as Choice, Library Journal, Publisher's Weekly, Booklist, etc. Reviews in these kinds of publications are not scholarly reviews.