Best paint for bookshelf

Paint basically does not cover imperfections at all.

Even with fairly "thick" (high viscosity paint), I would guess it would take at least three coats to even get a thumbtack hole filled in, and you'd probably still be able to see an indentation in the wall.

Depending on the size and nature of the "imperfection", you might be able to use spackle or skim coat. There are some primers on the market that can supposedly cover very small imperfections (like a scratch from sanding) but I'd be skeptical of that.

You can't cut corners here. It takes practice (and early on, lots of time and patience) to be good at applying it. You'll have to sand it afterwards. Use a flashlight and shine at an angle across the wall, and fix any bumps, ridges, etc before you prime or paint (it's significantly harder to fix after painting as you're basically starting the process over).

Sherwin Williams has a product called Armorseal Treadplex that I used on concrete and porch/patio floors. It's a strong and reliable water-based product. How well any paint performs after its applied will depend on the condition of the previous coating. I would scuff sand the previous coat with 150 grit and vacuum up all dust to prepare for top coat. Treadplex only comes in a Semi-Gloss sheen level. Because you're planning on using paint, I would not advise finishing the floor with a clear coat. The reason being if paint does chip or wear, you will be able to simply touch it up as needed.