I print books myself at home and have a lot of Amazon books lying around. What usually is the problem with Amazon printed books is that the author didn’t put in the extra time to get everything right. Professionally printed books for example use slightly gray letters on creme paper. Like for websites, this lowers the contrast and feels more natural for humans. Furthermore, many Amazon books are just poorly formatted. Text too big, margins too wide, cover misaligned with spine, text not justified properly, and things like that.

> Professionally printed books for example use slightly gray letters

This is simply an artefact of offset printing.

> Like for websites, this lowers the contrast and feels more natural for humans.

Text printed by an industrial laser printer on cream (or Natural Shade as it's called in the industry) paper looks discernibly crisper than what an offset printer produces.