How to decorate a big bookshelf?

You could store some of your art supplies in baskets or canvas storage totes made specifically for shelf units like yours. You can find them in most stores like Target, WalMart, Lowe's, etc. You could get coordinating storage boxes for your magazines. Space them out so they balance visually on the various shelves and then add small decorative pieces (a glass or wooden bowl to put small 'random stuff' in) and colorful scented candle(s). If you can see thru the shelf unit to the wall, you might like to add colorful fabric (tacked to the back of the frame) for more interest.