It cant go below the lowest brightness setting unless an app specifically has a function to dim the app rather than the screen.
What may be useful is going to Settings > General > Accessibility and setting Triple-click Home to Invert Colors.
Then pressing the home button three times in rapid succession will invert the screen, which generally makes things a lot dimmer and more comfortable to read in the dark. (when reading a book, I actually prefer to invert the sepia tone rather than black-on-white)