You can go into the iPhone/iPad
Settings-->Display & Brightness and try one or more of these options:
- selecting Dark Mode
- turning on True Tone
- turning on Night Shift
Personally I’ve never been a fan of the Books app. Apple begrudgingly included it, but it is obvious they don’t give a bleep about it or about their book store. Therefore it is a bare bones app with hardly any viewing options.
I have no issue reading for hours on my iPad mini 5 using the Kindle app, but I never read in a totally dark room at night. I always leave a very dim light on. I also have a Kindle Oasis 3rd gen., but I prefer to use the iPad mini 5 most of the time.