A bunch of shelves filled with stuff that does not need display is a far cry from sleek and sophisticated, so what are planning to display and honestly wall paper in the space does not sound sleek either You have a wall of panelleing and IMO those shelves should be removed and some nice art hung there Too many arches are just too many The room already has a lot going on without that shelving .

I like the shelving, and dont mind the adjusting holes either. I would just paint the interior rather than wallpaper it. A darker color on the back and side, would camouflage those holes, if they bother you that much. I would not paint the shelving itself.

I'd paint the total room including the shelving a dark sophisticated warm gray paint color like you see below.

Ceiling, bookself bookshelf, all walls?

Even if it was just the walls and shelving, a room all in that dark grey would be depressing and oppressive.

I bought a gorgeous wallpaper with a flower pattern for my living room at guipp-decor.com

Check their website! They have plenty of patterns to choose from

First - I apologize if my comment is a bit confusing and/or disorganized. I was in the hospital and am taking some medication that makes me a bit "goofy."

You could color drench the entire room - if the room works as a dark and moody style. I am going to do that to one of my rooms - I just need to select the color.

Once the bookshelves are painted, you won't notice the holes for adjusting the shelve as much - even if you don't fill them prior to painting (the darker the color, the less noticeable they will be).

I like having adjustable shelves - it's nice to break up the sizes of the shelves + customize them depending upon what you're displaying. If you really want to cover the holes - you could do the same thing as suggested above re: using foam board (just paint it and use it on the sides). You also could add some type of trim that would cover the holes - you could paint it + install it in a manner that makes it easy to remove when you want to change things in the future.

Using foam board for the back of the bookshelves makes it much easier to change the wallpaper in the future (or even the paint color if you left the bookshelves white and wanted to add a pop of color to the back of them. You could use peel and stick wallpaper for the back of the bookcases. The peel and stick wallpaper + using foam boards would make it an easier project + much easier to change in the future.

I'm sure that there are many ways of disguising the holes if you do a search. You could fill the holes with white caulk, white wood filler, and white DryDex spackle, a white wood fill stick by Varathane (however, if you are going to paint the bookcases - I would avoid the Varathane wood fill stick because it's waxy and will most likely cause paint adhesion issues).

Before you fill the holes, you need to decide if you will want to to have the ability to adjust the height of the shelves easily - some ways will make it more difficult to do that in the future - other ways will still allow you to easily push the shelf pins through when/if needed.