The more places you look for reviews,
the greater the variety of opinions you will get.
Start out by considering time...
You need to be aware of the years in which reviews were most likely to have been published. To do this, look inside the book for the COPYRIGHT date. (Don't be fooled by printing dates.) Look for something that looks like:
Copyright © 1990
For a 1990 copyright you would begin looking for reviews published in 1990. However, for most books, you can expect to find reviews published throughout 1991 and even into 1992 and 1993 and beyond, if it got reprinted and became popular again.
Next, decide where to begin looking . . .
There are essentially 4 types of indexes that will lead you to book reviews.
General Databases search articles in a wide variety of magazines and journals and a couple of newspapers. Some of these articles are book reviews! For books published since 1990, this can lead you to quite a variety of reviews.
Book Review Indexes/Databases will lead you to reviews published in a wide variety of periodicals and newspapers, a similar sampling to the category above, but especially critical if you are looking for reviews of older books.
Subject Indexes/Databases index professional magazines, scholarly journals and other literature in a specific field or academic discipline. You can use these to find reviews from the perspective of a specific field. Some of these same magazines and journals are covered in General Databases for more recent books.
Newspaper Indexes/Databases are the most challenging to search! Although there are a couple of newspapers that could be considered broadly American, like the New York Times or the Washington Post, this is where you can look for reviews with a geographic perspective. Our major newspaper databases all include foreign newspapers. With only a couple of exceptions, though, these databases will only yield reviews published in the 1990s and forward.