Shingling or creep occurs when pages are added to saddle stitched books. The inner pages get narrower as more pages are added (nested) and depending on the thickness of the paper. This can cause elements to be cut off in the final trim.
How do we compensate for this?
By bringing the elements of the page in towards the fold one sheet thickness at a time. First sheet will be creeped in by the thickness of one sheet and the last sheet will be multiplied by sheets per thickness.
We use software to compensate for creep. It is something to keep in mind when laying out a book with many pages.