A common mistake we see is when authors place their content too close to the edge of the page or cover, where it's at risk of being trimmed off during production.
Each of our bookmaking programs offers trim guidelines to help you see approximately where the trimming will take place. For information specific to your project please follow the link below for whichever program you're using:
Trim guides in Blurb's InDesign Plug-in
Trim guides for the PDF uploader preview