"I have more books."
Translation:Jeg har flere bøker.
In English we use "more" for countable and uncountable nouns. "I should drink more water" and "I have more books." But we use "fewer" for countable nouns and "less" for uncountable nouns. "I should drink less water" and "I have fewer books." Norwegian has two words (flere/mer) to mark this distinction whereas English only has "more."
I am confused as well, but I think DanielHeck2 made the best point above. In English there are countable and uncountable nouns. As he said "I drink less water" "I have fewer books." We have separate words for reduction of a noun whether countable or uncountable. Apparently Norwegian has separate words for increasing the countable or uncountable noun.