Did you receive a pop-up when uploading your book notifying you that one or more of your images contains transparency? This means that you have probably placed either a PNG or TIFF file in your project. Note that any transparent parts of the image will be filled with white when rendered.
You can test any affected pages by exporting your book to PDF via the Export Book to PDF option located to the lower left of the Book Module. Review the exported PDF to look for any problems.
To fix an image that has been flattened with white:
- Go to the source image.
- Flatten and re-save it, or save as a JPG (which will automatically flatten the content).
- Re-import any flattened JPG files into Lightroom, then into your book.