Your books are printed by an HP Indigo printing press, which uses liquid ink technology that is similar to offset printing. Other digital machines use toner, which sit on the surface of the paper, meaning images may appear glossy.
We don’t modify your images at all, but do convert them from RGB to CMYK for print.
We recommend converting your images to sRGB IEC61966-2.1 color profile for true results. It’s always worth viewing your images in CMYK in Photoshop to gauge the color difference. The CMYK color profile we use is ISO Coated V2. Previewing your images in this profile will show the expected change in color from sRGB to CMYK.
Please note that images printed with the HP Indigo do not hide the texture or surface of the paper, as you might see with some toner-based prints. Instead the ink absorbs into the paper somewhat, which is similar to what you would see with traditional offset lithography.