If you have an image that you want to print to the edge of the book, then that image “bleeds”. This is often done on book covers. For the printer to be able to trim the books so that the image is at the edge there must be some part of the image that gets trimmed off (or else you will have a white stripe of the paper showing). The amount that gets trimmed off is the “bleed”, and printers require a minimum of .125″ (.25″ is preferable especially for children’s book pages). So, be sure that you set up your files so that you have enough image to go beyond your trim. In other words, a 6 x 9″ cover that bleeds all 3 sides on the front will really be a minimum of 6-1/8 x 9-1/4″. And a children’s book that is 8 x 8 will have pages that are 8-1/2 x 8-1/2″.