Book reviews appear in academic journals and give scholars a good idea of the value of upcoming books. They often list the strengths and weaknesses of both the writing and the research, and will give you an idea of where the book fits into the scholarship.
Book reviews are generally written by other scholars in the field who are a good judge of the history being explored.
In Ebsco databases like America History & Life or Historical Abstracts, choose 'Book Review' from the 'Document Type' box.
You can tell that a result is a book review by looking at the icon to the left, where it lists 'Review' instead of 'Academic Journal' or another label.
In JSTOR, choose 'Reviews' under 'Narrow By'.
You can tell that a result is a book review by the bolded label to the left, which reads 'Review'.