Open your book project critique with a detailed explanation and analysis of all the excellent components of the project idea. For example, if the author has included a detailed bibliography, praise her on the selection of research items. Similarly, if the author has presented you with several of the conclusions she hopes to reach in the book, highlight the significance and power or several of these conclusions. By opening your critique positively, you in turn open your author to constructive criticism, as she will understand this criticism to be supportive, rather than merely critical.
Transition to your constructive criticism segment with a statement that frames the criticism in a supportive light. For example, "In order to strengthen your third conclusion, consider the following points," or, "To build upon the power of this argument, supplement your research with the following sources."
List your critical points in concise, clear bullets. This makes it easier for the author to come back to your advice as she works on her book project.
Close your critique with a supportive and encouraging statement conveying your enthusiasm to see how the author is able to respond to your comments.