Return it, take measurements of the perfect size and then go shop :) And buy one that is much taller.

I also say return it but before you do measure to see if the bookcase will fit where you now have the tree, between the window and the wall. Might be too big but if there is enough room, try it there and put the tree where the bookcase is now, in the center of the back wall, snd think about what art might look nice around the tree. It would be unusual but it could look classy with the right balance of art.

Your home is lovely and looking at the photos closer, if you could skootch your sofa up a bit, you might have the room to put a statement armoire piece on that wall. I think it would serve your towel purpose plus more things you might want to hide and add more of a statement than a bookcase.

Ideas: plants, small pictures, books, plants and a large round mirror.

Mirror add height, plants do, too. Move the book case closer to the faux floor plant, as in second photo. To fill the remains wall space, add a chair and a skinny floor lamp.

Or, this arrangement on a wall that looks similar in size to your space.

Notice how the pottery is adding height and the the 4 botanicals and mirror hung even higher. This room is gorgeous, as is yours. And the view over the back of the sofas is so similar.

Thank you for the ideas so far, they have been very helpful. Going to move around the bookcase and tape out a few art pieces to test these looks out!