The exact meaning of a “book review” may differ based on the level of audience for which it is written. A middle school book review will probably look a little different from a book review that a university professor would write, but even so, there are sure to be some commonalities. The general structure of a book review is the following:
1) A rich description of the book: What are the contents? How are the chapters broken up and arranged? What methods and what types of sources are used (if it’s research-type book)? What does the book set out to do? and so on. This section would not replace an actual reading of the book, but it would give the reader of the book review a clear sense of whether or not the book would interest them.
2) A developed evaluative statement about the book (maybe at least 1/3 as long as the first section): How does this book fit within or work against a tradition or trend? How compelling is the writing (especially if it is a creative work) or the argument (especially if it is a critical work)? Does the book succeed in doing what it set out to do? and so on.
The link http://teacher.scholastic.com/writewit/bookrev/read.htm given below gives example of perhaps a middle-school-level book review. See other sections of this site (e.g. “Writing Tips”) for more information, if this example matches what you are looking for. Don't necessarily stop here, though!
A second link leads to a pdf file with good advice on writing academic book reviews:
Enotes has some very useful information, too, of course: http://www.enotes.com/topic/Book_review
I hope that these comments help!