A book review, when written by a professional, is intended to assess the quality of a work of literature, and may include technical as well as stylistic assessments. For example, a reviewer may observe that a writer fails to develop a certain character sufficiently for their story, or that they use plot devices that are predictable.
Educators who assign book reviews to students generally do so to get them to critically read a work of literature. A typical book review of any sort usually involves a brief summary or synopsis of the material in the book. Often a reviewer would analyze the content to determine the themes an author is interested in working through, and to discuss the characters in the book.
This analysis would be followed by the types of assessment or evaluation mentioned above. How well were the characters developed? How effective was the author in bringing the themes to light? Finally, some book reviews use other, similar works to use as a reference point for a work of literature. This could be other works by the same author, or different books that engage with similar themes.