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 book-making tools offers trim guidelines to help you see approximately where the trimming will take place. For information specific to your project please review details for whichever tool you are using:
Trim guides in Blurb's InDesign Plug-in
Trim guides for Blurb's online book-making tool, Bookify