First, have a look at where in the room they are. If you have valuable books, you want to avoid direct exposure to sunlight. Also, look at the lines the shelves create. Would the room look wider if the shelves were opposite the entrance instead of along the side walls? Could you arrange them around a corner with a chair to create a reading nook?
Once you're sure they're in the best possible place, dust them. Dust them like crazy. Dust them for real; take the books off the shelves and rag-dust.
Look at the books on the table or floor, wherever you've put them. Do you need all those? There's probably a stack you haven't touched in years. Is it an attractive stack of books? Unattractive books you don't ever use should go into storage.
Once you've got clean shelves and only the books you need, try arranging them by color. You can create all kinds of patterns; a rainbow, alternating chunks of reds and blues, lights and darks. Play around with stacking, as well. Lay some books horizontally in stacks to create bookends, or in the middle of a shelf, top a stack with a trinket.
You have nothing to lose; this is one of those rare decorating choices which is completely reversible. Why not try it?
It's just another easy way to make life more lux for cheap or free.