If you add images to a Book using the Autoflow Unplaced Images option, Aperture may create books that exceed the 99 page limit for book orders. When you try to order the book, a dialog warns you that there are too many pages and advises deleting some images.
Aperture allows this so that you can create books of whatever size you like for self printing. If you intend to order a Book, be sure to keep the number of pages at or below the 99 page limit.