Book reviews can be found using a number of different resources.
JSTOR excels at providing critical book reviews. When searching for reviews in JSTOR be sure to utilize the "Review" filter.
Look for this type of filter in other databases such as Academic Search Complete.
Critical reviews can also be found in major newspapers like the New York Times.
The Libraries' InfoKat Discovery is a great tool, too, for locating book reviews in all disciplines. When searching in InfoKat Discovery for book reviews, be sure to refine your search results under "Resource Type" to "Reviews."