All the photos you've selected to use in your book project appear in the photo well. Any photo that is already used in the book has a blue check mark on it. If you remove a photo from your book, it is unchecked in the well, indicating that it is not used in the book. For example, the right photo in 2nd row is unchecked in this image. You can select HIDE USED PHOTOS from the OPTIONS menu in the well to show only the photos you haven't used (see left).
To remove a photo from the well (and the project) you can click on X on the photo in the well. If the photo is in use, you'll be prompted to first remove it from the page where it is used. If it is not being used it will be removed from the project.