The only bookcase that can be used "as is" is the fir one. To use other bookcases to hold books you need a shelf. Put the bookcase in place, then line the shelf up with the shelf of the bookcase, place books on shelf, rinse and repeat.
The bookshelves look nice, but so far I hae been unable to place any books on it in my room. Just a note for those of you who get this reward and plan on sprucing your room up a bit with it.