Creating book w/Pages. Export to PDF images are sepia

Apple's PDF export does not support PDF/A or PDF/X standards, or Spot colors in PDF. LibreOffice Writer supports PDF/A-3 or PDF/UA-1 export. It's about the PDF library used and you can get the first sentence PDF standards best by using Adobe Acrobat Pro, or constructing the entire book in Affinity Publisher where you have absolute control over the color space and the PDF/A or PDF/X export features.

You can review the PDF content criteria for PDF/A or PDF/X. Transparency is only supported in PDF/A-2 or later. More from the PDF Association.

In Pages, everything is treated as though it is in the RGB colorspace. Clearly, there is no transparency in JPG images, but JPEG-2000 (JP2) does support transparency as does PNG, TIFF, and PSD content. I have content in all of these formats and when added to a Pages document, even overlaying two PSD files with transparency, I cannot generate a PDF with any color shift, or to sepia as you mention. That sounds like something going on with Photoshop.

Apple's PDF export from Pages will embed used font subsets, but not the entire font family, and images are certainly included in the exported PDF.

The Pages document background is not transparent. By default text boxes and unfilled shapes have transparent backgrounds in Pages. Specific artwork that has a transparent background retains that quaility unless your export to PDF/A is not PDF/A-2 or greater, or something else in Adobe specs interferes with transparency retention.