You need to make sure you observe the recommended clearances around the firebox, the amount of space above and on either side of the firebox that must be composed of fireproof material like brick, slate, marble or stone, or some other noncombustible material. If your fireplace is a prefab, the manufacturer's owner's manual should give you the clearances. If your fireplace was custom built, you could ask a chimney sweep for his recommendations about safe clearances. If it was properly designed and built, it already has noncombustible material around the firebox. So your only concern is not to disturb that, or to replace it or cover it with another noncombustible material for decorative purposes. Just make sure your bookcases are outside the clearances.

By the way, if you haven't cleaned your chimney, you should do it soon, while chemney sweeps are idle. If you don't know one, ask around for recommendations from friends or neighbors.

Good luck with your plans.