It is creeping out of the spine of your book. No we aren't talking about a ghost in an antique book.  We are of course talking about page creep.  As you fold more and more pages together into a thicker book, the thickness of the paper starts adding up. To compensate you have to shift each page further away from the spine of the book.

When setting up your print file you don't need to worry about adjusting for creep.  We will do that for you. You should however be aware of what creep is and how it can visually impact your final design.  For example, lets say you have a spread in your book and a title that spans across two pages.  The middle letter of that title is "W" and it sits right on the joint.  When you disassemble the book into its component parts each half of that page spread is on a separate sheet of paper.  If creep has not been accounted for properly the middle of that "W" may be hidden in the spine of the book and you will visually get a "V" instead. 

Pages in the middle of the book (or signature in the book will be pushed out by the thickness of the paper in the spine.

In addition to the spine of the book, creep can also effect the outside edges of the page.  One way you can see this is to flip through a book and watch the page numbers.  If the position of the page numbers appears to shift out and in, then the printer has not properly accounted for creep.