Styling bookshelves can be daunting!! You need to tackle it kind of like a puzzle, start with a clean space and then have a few larger items along with some books. I personally love decorating with old books. I am obsessed with them and pick them up whenever I am lucky enough to run across some. You can use any books, you really can’t go wrong with them. If they are not a color I want I have even been known to paint them. You can also vary their directions on the shelves, horizontal or vertical or even stacking where the pages show.
Some of the larger items you might want to consider are pottery pieces, boxes (functional to store things in) art/sign, spheres, things that might have a unique shape.
No matter what your design style these items can be found with your look and style. You will also want to add some green or botanicals not a lot but just some to add a little softness. All of these items will give you a variety of sizes, color, and texture.
Another item of interest you may or may not want to add are simplistic coordinating picture frames to tell your story.
Start to layer your heights on the shelves, don’t be afraid to adjust your shelves to vary the look and height of them. Place taller items on the shelves with more height and you can always place an item on a stack of books or a box to take up some of the room. Break up some of your books with some of the other larger items you have. My rule of thumb is always less is more so add your objects with giving your shelves enough to make them interesting without overdoing.