Is there a way to build up your mantle and surround so that it “connects” to the bookcases and looks like one solid built in? I don’t think white will work with you brick, but maybe another color.

If that is a focal point and you are not adverse to painting the brick, I'd paint everything a mid-tone and then the back of the shelves a couple of colors darker on the same color strip. If you're leaving the brick as is I'd probably paint the rest of that wall and wood in the color of the grout in the brick. Then maybe change the mantle and paint out or replace the brass fireplace screen.

Wish the photo would enlarge so we could see details.

Depends on the rest of your furniture and the natural light of the room. Do you need white paint to bounce more light into the room? Would a darker wood tone work better with the furniture? A couple coats of a darker poly and stain like Minwax Polyshades would be easier than sanding priming and painting to get a nice painted finish. But a warm white would work with your brick. If you have matching oak furniture try some removable wallpaper to back inside shelves to lighten it.

I have a medium warm brown brick fireplace and originally it had brown stained built ins. In a moment of poor judgment I painted them white. I really wish I had the original stain back.

Is all the trim in your home stained? Unless you have other white trim, I would leave the bookcases as they are. I would remove that mantle and the odd square blocks around the fireplace. Get a longer mantle that connects the two bookcases, and hang it lower than the current one. Or do without a mantle and use larger artwork above the fireplace. I would remove the brown crown molding, or paint it the same white as your ceiling if it can't be removed.