BOOKS: (because… duh)- different heights, but important to have the same depth so they all flush together when stacked on the shelf. If you are aiming for a more chic aesthetic select books that have a similar color palette or theme like travel books, fashion, history, etc. Lay some books flat and place small decor on top of the flat books for variations of height. Lean other books against the ends of the shelves with bookends for support, but also design.
CANDLES: Real candles or battery operated are also fine! Having some illumination in the evening hours brings a romantic aesthetic to a book shelf. Also… who doesn’t love candles.
FRAMES: Tilt them a little inward so it brings your eye in to the center of the shelf. I prefer different mix & match frames, but matching frames is also nice for a cleaner, simpler design.