There are many different sources for book reviews.  First check the date of publication of the book.  If it was published more than 4-5 years ago, JSTOR is a good starting place.  JSTOR does not include the most recent issues of journals, so if the book was published in the last 4-5 years, you should try another source, such as Project Muse.  Historical Abstracts also contains citations to book reviews for recent, as well as older, books.  Multidisciplinary databases, such as Expanded Academic ASAP or Academic Search Complete, are also a good source for books published in the last 10-15 years.