If you swipe down from the upper-right corner of the screen, there's a brightness slider you can use to adjust the screen on the fly, I always drop it down at night in bed - if you disable auto-brightness it gets pretty darn dark, I couldn't imagine that being too bright. Also, if you do a long-touch on the brightness slider there, you can turn on Night Shift, which lowers the amount of blue tint on the screen.
To disable auto-brightness, go into Settings->Accessibility->Display & Text Size->Auto-Brightness (last option).
For me personally however, I dislike reading books on any sort of backlit display at any time of day, it's easier on my eyes to read with an e-ink display like on Kindle. But I browse the web in bed on my iPad Pro all the time.