Turning on your interior light in your car is not illegal in most places and it's okay to turn the interior light on for awhile as long as it's not blinding you by making it hard to see other vehicles ahead of you.
When I travel at night I will turn the interior light on when I need to glance at directions that I keep on my dash just to keep the directions fresh in my mind.
I've never been ticketed for the interior light nor have I been pulled over for the interior light on while driving at night so I would say you're okay.
Just don't turn the interior light on and try to read a map while driving because that can be very distracting and could cause a car crash.
If you need to really look up directions and have no passenger with you who can look at the map or look up directions on a tablet or laptop then pull over, turn your interior light on and then look up the directions.