Ksiazki is referring to multiple books if I am not mistaken.... Therefore its saying that we need books.
If I was to tell someone I wanted a single book, I would say "potrzebujem ksiazke". Its the way I learned it and calling a single book ksiazki just feels unnatural to me. But, I guess you learn something new everyday. Thanks! I appreciate the input/reply. Edit: Alrighty, I see the reasoning now but still looks unnatural to me! :P
'I need a book' = 'Potrzebuję książki'
You really need a genitive after 'potrzebować', so 'potrzebuję książkę' is incorrect.
It is a mistake commonly made by native speakers (using accusative instead of genitive after potrzebować). It does not mean it's right but it could be why David learnt it.
I'm afraid you began to make the same mistake all over again. So let me remind you: almost all feminine nouns have the three following forms identical: Genitive singular, Nominative plural, Accusative plural. The verb "potrzebować" takes Genitive. Therefore this instance of "książki" must be Genitive singular.
Genitive plural (We need books) is "książek". "Potrzebujemy książek".
I guess it was ingrained in my formative years and is difficult to accept the change. We call that a bonehead. :-)