"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.
Because you use plural suffix (for objects) only when the object is the direct definite object, not when the object is indefinite. If we would say "kitapları" that would be correct, but in this sentence, the books are not direct definite object thus they won't receive any plural suffix.