If you don't see your subject or discipline in the list below, try Academic Search Complete--it is usually a great place to start.
Use this quick search to search within a variety of review sources. TIP: search for the author's name, the book to be reviewed, then limit to "reviews" after you search.
Look for reviews with substance. If you can find one that is a page or longer, it probably gives you a more in-depth reading and analysis than the short ones from Publisher's Weekly.