I would definitely paint it.…..unless you like the look of pine wood grain. its hard to make a decision without seeing the space and your decor.

Is pine wood grain different from other wood grains? I like seeing the wood grain but only if it is classy looking and not “cheap” looking.

When you paint pine be sure to properly prime the knots so they do not bleed resin.

The current pine offered takes stain terribly. If you want to stain the bookcase it should have been made from plywood veneers.

If you use a really dark stain on pine you might end up with an acceptable outcome. BTW you have to condition the wood first before you stain it.

If I did paint on the pine wood, what kind of paint and what covering should we do if we want it feeling shiny and smooth?

There are a number of different paints you can choose, but this would work for acrylic:

sand it, prime with Killz 2, let dry, fill any imperfections with joint compound, let dry, second coat of Kilz if needed, let dry, lightly sand 220 grit, vacuum clean, first coat acrylic paint with new brush, let dry, second coat with new brush, let dry a week before using.

Does the painting happen before the bookcase is built or after it’s installed?

That depends. If the shelves are removable you would take them out to paint. You could paint the inside of the case when the shelves were not in the way. Other than that, you talk to your carpenter.

I would use BIN primer. Original BIN, pigmented shellac, red label. PPG's Breakthrough paint.

BIN sands very easily. Fine dust & doesn't gum up the sandpaper. Breakthrough paint dries very fast and is a great durable, blocking resistant paint.

Blocking resistance is the ability of a paint, film, or wrap to avoid detrimental adhesion between two surfaces—to not stick to itself or another surface upon contact when pressure is applied.