A book review is highly subjective (based upon personal feelings, emotions, and preferences) and provides the opinions of the reader. A book review's focus lies in a critical analysis and evaluation of a text (specifically focusing upon strengths and weaknesses of the author, the book's purpose, and an evaluation of the author's success regarding the book's purpose).
In regards to writing a book review on Natalie Babbitt's Tuck Everlasting, one could focus upon numerous areas of the text. The typical aspects of a text which a book review can focus upon are as follows: character development, narrative voice, syntax (arrangement of sentences), rhetorical devices (proper and creative use), and presentation of theme/message.
For example, one could examine the development of Winnie Foster over the course of the text. A review will focus upon who she is (in the beginning and end), how she is characterized, if the characterization provides a well-developed and defined character (is she believable?), and if she is dynamic (changing) or static (stays the same, no growth).
Another focus for a book review will focus upon the narrative voice. The text possesses a third person narrator. The review will examine how effective this narrative voice is at telling the story. Readers will decide if the narrator is reliable or unreliable and if the narrator makes the story easy to follow or confuses the reader.
One final example would require the reader to examine Babbitt's syntax. Do her word and sentence choices support the targeted audience? Does she make the language too hard? Does the language mirror her characters and what is happening in the text?
Be sure not to provide a summary of the text. A book review does not summarize a text. Instead, use textual examples to support what is being stated.