My understanding is that English plurals are not always translated into reduplicated Indonesian words. Are not people more likely to say "Kami membaca koran dan buku"?
In everyday speech, yes. People only duplicate the nouns if they explicitly want to state there is more than one item. It's normal to just tell by the context, though.
Would "surat kabar" be an appropriate substitue for "koran"? I've never heard that word before :))
"surat kabar" isn't used as often as "koran" in daily coversations, but it does mean "newspaper". I'd say "surat kabar" is more formal but it would also be acceptable to use "koran" in formal speeches.
Why not koran koran?