"You do not need books."
Translation:Jullie hebben geen boeken nodig.
My gut says if you used that word it would have to be "Jij hebt boeken niet nodig", but don't quote me on that. Let's hope someone comes along who can explain.
If I had to summarize, I'd say "niet" is a general negation of an entire sentence, while "geen" is a negation of a specific noun - basically, the opposite of "een", but also used in the plural.
Here's a longer explanation: https://www.duolingo.com/comment/3734833 Check out this one.