Sometimes an image may appear in a book layout, but not in the book's Browser.
This is the expected behavior if you add an image to a book, then later you return to the project and (without thinking about the book) stack the image in question with another image that is not in the book's set of images.
If the image in the book is not the pick of the stack to which you added it, the result is that neither the stack nor the book image appear in the book's Browser, but the image will still appear in the book's layout. If you make that image the pick of the stack, then the stack appears in the book's layout.
If you don't want to worry about this, just remember to avoid stacking images after they are placed in a book.
This issue is resolved in Aperture 2.x.