The most important thing in writing a book review is to be honest and keep it brief.  You do not need to bash the book or sing its praises.  Stay generally in the middle, but honestly describe the good and bad things about it. 

Some of the things to include in a book review are the title, author, publisher, medium (is it online only, for example), length, and reading level.  Most of Spinelli’s books are for kids.  This one is no exception.  So you’ll want to choose an audience: kids, parents, or teachers.

Once you have chosen an audience, some basic elements to include are a description of the main characters (such as David and Primrose), a summary, a quote, and a list of positive and negative points.  For example, you might consider the book funny or depressing.

When choosing a quote, you will want to consider an important thematic element.  For example, consider David’s sadness about his mother’s death.

He had promised himself he would never see the sunrise without her.  Along with obeying rules, he hoped this would bring her back. (ch 13, p. 67)

This quote is touching and to the point, and it hits on the major theme of dealing with loss.

I like to not give away the ending in a review, but it's up to you.

Finally, try reading some other reviews.  Don’t read one for this book, so it won’t impact your judgment, but read examples.