A book talk is like a movie trailer. The purpose of a booktalk is to "sell" the book. You want to give enough of the plot to interest the students, but you are not giving a summary of the book. You don't want to give away the important parts of the book, and you certainly never want to give away the ending. You want to highlight the interesting points. You may want to read certain passages to your students. The primary purpose of a booktalk is to grab the audience's interest and make them want to read the book. It's always a good idea to end the booktalk with a cliffhanger. It is also good idea to have the book as a visual prop.
(From Nancy Keane’s Children’s Literature website )