"These books are new."
Translation:Ces livres sont neufs.
"Nouveau means new in the sense of new to the owner - a change or improvement; that is, something that is new because it's different than what came before, regardless of whether it is brand new from the store."
"Neuf means new in the sense of brand new, fresh out of the factory, first of its kind."
'quoi de neuf ?' = 'what's up?'/'what's new?'/'how's it going?'
I believe it is the difference between "recent" and "new". We could have new copies of ROBINSON CRUSOE, which is not a recent book at all. Hope that helps.
I think it's because Cette is for feminine nouns and le livre is masculine
"Ce" is invariable when used by itself as the subject e.g. "Ce sont neufs." [These/those are new]; when describing a plural noun it becomes "ces" e.g. "Ces livres sont neufs."