timbre matching as the vets have mentioned. same make, same drivers will be the best.
do you already have the speakers?
If you are going to purposefully go out and buy new then just get an LCR set. Rids the hassle of center placement with bookshelves.
If you have an extra single bookshelf that you want to use, then just use it.
If it does not sound right, then you will know what it is you are actually looking for - soundwise.