Scholarly book reviews range in length but are almost always written by scholars or experts in a field and published in a peer reviewed journal within the same field of study. These reviews help determine if a reader or researcher should use the book in question as a part of their research, and can sometimes be used to determine if a library or scholarly institution should purchase the book. These reviews often contain critiques of the research material from an expert perspective and can be useful secondary sources in their own right.
These databases are great places to start looking for scholarly book reviews, but keep in mind that many subject specific databases contain journals that may publish a review occasionally even if it's not a regular part of the journal.
When you're doing your research look for a "format" or "document type" drop down or list and see if reviews are listed!