Reference works are a great way to find general information about a topic or to help you find a topic for a research paper. Finding reference works such as dictionaries, encyclopedias, or bibliographies in IUCAT is easy!
Scholarly book reviews are often located in the same journals in which scholarly articles appear, therefore begin your search within the same article indexing databases. In the majority of these search interfaces you can go to Advanced Search and limit to "Reviews." Alternatively you can add the term "review" to your keyword search query.
There are also several databases that are specifically for book reviews. Understand that the reviews you find here may or may not be written by historians or professionals in the history field. For example, librarians review a history reference work but have different criteria for reviewing than a professional historian would.
Understanding more about a journal or published helps researchers determine the focus, context, and possible bias or value of a given publication/set of information. Information about journals can be gleened from many locations.