Layer 3: Interest

By now, you should have the functionality of your bookcase covered.  Presumably, it houses books and may even store some other household items.  Now it's time for the fun layer: personal and decorative items.

Really, anything goes here in terms of the items you choose.  Except for candles, which aren't safe to burn in a bookcase (unless they are battery-operated, of course), you can choose a variety of objects to display as long as you follow three basic rules:

1. Have a Common Theme--Make sure most of your decorative items share something.  Link your items with a repeating element such as color (e.g. items in shades of yellow and gray), theme (e.g. nautical items) or type of object (e.g. silver picture frames).  This will lend cohesiveness to your display and prevent it from looking like a hodge-podge of "stuff."
2. Balance Your Collection--If you add in an unexpected pop of color, try to repeat that color somewhere else on the opposite side of the bookcase.  The same rules apply for items of different sizes or shapes.  
3. Vary the Scale--Keeping balance in mind, it's also important to vary the scale of your objects.  Try to avoid displaying lots of tiny objects.