You are wise to think of this first and before the book is published.  You should seek help from an attorney in your area as to drawing up a release for the photo participants to sign and make sure that they do indeed sign it before you publish their picture.  Everyone thinks how great it will be to be in a book, until they are the "don't" picture in the do's and don'ts.  Or they look "too fat."  Make sure that if you do use their photo to represent something that could be construed as negative that they understand that it is educational and meant to be critiqued.  And maybe protect their image in some way from being a butt of criticism by blocking out their face, etc.  Your ability to alter the picture in that way make need to be on the release as well. Good luck.