Reading book reviews can help you decide whether or not a particular book is worth your time, and also can help you put the author's work in context and suggest other places to look.
Easiest: if you are looking for a review of a book published in the last 20 years or so -- use the "All" search box on the library home page and search for keywords from the title, the author's last name, and the word "review"
The next easiest place to find reviews of scholarly books on American history is America: History and Life, which indexes the book reviews published in scholarly journals on American history. (Unfortunately, Historical Abstracts does not include book reviews.) If you are looking for a review of a book published after 1995, you should also search H-Net.
For more approaches, see the web guide to Finding book reviews in history.