"The Greeks are reading books in the church."
Translation:Yunanlar kilisede kitap okuyor.
I think if you use a plural ending, you have to also use the accusative ending ("kitapları"). But if you do that, the direct object becomes definite, and the sentence becomes, "the Greeks are reading the books in the church."
In this case, the English sentence is just talking about books in general, so "kitap" will be a general direct object, and won't take either a plural or an accusative ending.