In which book are they interested?
Ok, "books" instead "book":-)
Could this also be "Which books interest them?" or is there another way that this is said in Duolingo?
'Which books interest them?' = Které knihy je zajímají?
Why only "books" is right? What about "book"? Knihy - that's books and a book, isn't that?
A single "which book" in the nominative case is "která kniha", in the accusative case with "o" it is "o kterou knihu".
So "O ktere knihy" is in the accusative..? As a rule, if something follows a preposition, would it be in the accusative?
No, different prepositions have different cases.