AlecK, DerStrom8:
You say the speakers are built into the bookshelves - how many are there? Are there others in the house? Do you know the condition of any of them? DerStrom8 might be correct here, that they are just bare wires for connection into spring terminals - but if you only have a pair of speakers in a single bookshelf and no others, then you will have a bit of investigation to do. Something to keep in mind is maybe this was a multi-room speaker installation (are there any weird plates on the wall in the rooms - blank plates, or plates with terminals of any sorts)? You might have the beginnings of a very sweet multi-room setup, once you trace the wires.