"Book or bread?"
Translation:Könyvet vagy kenyeret?
It is a question. Like what do you want (a) book or (a loaf of) bread? They are subjects. It should be a question in Hungarian as well, like "Könyvet vagy kenyeret (kérsz)?" and in that case it is unambiguous why accusative form is used. The question mark should help a little bit.
(It is not easy to imagine a situation where a question is ever asked "Könyv vagy kenyér?". The only one I could come up with is still pretty unnatural. Even if I wanted to ask "What is this, a book or a loaf of bread?" I would say "Mi ez, egy könyv vagy egy kenyér?", so I would use indefinite articles.)
It is misleading, you can only make an intelligent guess that it is going to be the same translation now from English to Hungarian as it was before. Everybody will make a mistake here.
This is an English speaker's guess: for the same reason people say, "jó napoT," Boldog karacsonyT," etc. It's part of a larger thought, like "jó napot kívanok" (I wish you a good day") where "a good day" is the object of the sentence. Maybe it's the object of "a jug of wine, a loaf of bread, a book, and thou"......by Keats?