Finding book reviews is *not* easy.
It's particularly difficult to find a VARIETY of reviews.
So, read this guide very carefully and thoroughly!
Reviews Come in Many Types
The earliest reviews will be the briefest, often only a paragraph, and will be in publishing and library trade magazines like: Publishers Weekly, Booklist, Library Journal, Choice.
Close behind in time are longer and more substantial reviews in daily newspapers, weekly, biweekly, monthly or bimonthly popular magazine and journals of news and commentary like: Time, Newsweek, New York Times Book Review, Times Literary Supplement, Women's Review of Books, New York Review of Books, America, New Yorker, Atlantic, National Review, Commentary, New Republic, American Spectator, and so on.
Also right on the heels of reviews in these newsy sources are reviews in professional magazines. They will review books of particular interest to people working in a certain field. These include magazines like Science Teacher, HR Magazine, Physics Today, RN, Principal, New Scientist, Cineaste.
Last, but not least, are the scholarly reviews in specialized monthly and quarterly journals, like Journal of American History, Contemporary Psychology, Science, Journal of Popular Culture, American Literature, Signs.
Consider Your Book's Audience
The kinds of reviews you find will depend on the audience for the book. Books that appeal to the broadest audience, the general public of strictly the USA Today and People Magazine crowd, will only be reviewed in popular magazines and newspapers. Books that appeal to people who read journals of commentary/opinion will be reviewed in these types of journals. The American Spectator, for example, is a conservative commentary/opinion magazine. You might expect them to review a biography of an American politician, for example. And, the review would, in a sense, be a reflection of the opinion of politically conservative Americans. The same book could get reviewed in a liberal opinion magazine, like The Nation, and present a very different perspective. The same is true of trade/industry journals. In a way, these magazines speak for a specific group of people linked to a particular job or profession.
Scholarly books are reviewed in scholarly journals. There are even scholarly journals dedicated to just reviewing the literature of that particular field.