Caulk would be the quickest, easiest. But, yes, a trim moulding crown to finish it off, but then again, you'd probably need to caulk and paint it. But I don't think you have room above the lights for any additional trim.

You should use a small trim (maybe cove). It will give enough to eliminate the gap. You can caulk it, but depending on your climate and seasonal humidiy changes in your home, the caulk might pull away.

Not all caulk is equal! The best one I've ever used is Top Gun 400, also sold as All Pro, or Sherwin Williams carries something similar.

There's no actual problem here other than a cosmetic one. Caulking won't look good in the long run. Even if you do a pro job, getting the color to match is hard and for the color not to yellow over time. Crown molding is common for exactly this kind of reason. It hides this kind of imperfection.

It appears the ceiling has texture. One option is to fill the area of the ceiling only with Durabond 90. A very hard joint compound that dries quickly. First tape off the wall edge with 3/4" painters tape. Apply pressure to the tape near the top of the wallboard. Mix some compound and use a taping knife to fill the gap. Use a wet 1-1/2" brush to stipple it and move on quickly. Carefully remove the tape while the compound is still wet. When the job is dry retape the wall and paint the compound with paint to match the ceiling, blending it in. Remove the tape. If you experience separation in the future tape the wall and caulk. There shouldn't be as large a gap.

Scrape that popcorn ceiling.
It needs to go. And, it’ll probably make that space less obvious

Caulk is the only option and the best option. Not enough room above the light fixtures for cove molding. I second the recommendation for Top Gun caulk. I get it from a PPG paint store. (Pittsburgh Paints)

Big Stretch is a good alternative to Top Gun. Top Gun is more user friendly--tools and cleans up easier than Big Stretch.

The word is caulking and no way would I do that in that tiny space because it first is a tricky area to get perfect and as mentioned can discolor over time . I tiny piece of trim could be the answer but it is impossible to know how much space you have behind the lights .BTW the ceiling light is all wrong if the other lights by the unit are staying . Something like this to match the cabinet I agree that ceiling should have been dealt with but probably not going to happen now

A flat cut molding would best match the lines of the bookshelves. Is it best to caulk then put moulding to ensure white behind?
In case the moulding just fits. Could you do a wider moulding and cut around the lights if needed? Not a builder, curious DIY-er.

If you could take the lights down, then you could add a flat stock moulding (painted same as bookcase) across the top and pre-cut holes for the two lights to be remounted. If the top of bookcase is say 5 inches you could put 4" flat stock and a 1" decorative cove moulding beneath it or leave it without.

(Caulk)...can discolor over time.... getting the color to match is hard and for the color not to yellow over time.

Which is why you buy paintable caulk. Top Gun & Big Stretch are paintable caulks. Neither of them are prone to yellowing in any event. But you are going to need to paint it just like you would need to paint any moulding that you install there.

Caulk is easy, fast and inexpensive. May need some backer rod where the gap is widest. Damn, it is worth trying. Not like you can't go to a whole lot more work to move lights, add moulding, caulk, prime, paint if you don't like the result from just caulking.

Whoever was the somebody that installed that should have scribed that top piece to the ceiling. and you wouldn't have this issue. Actually, scribing is still an option if you want to take a couple of steps back.

Here is a YouTube link on how to scribe a valance to the ceiling: https://www.youtube.com/watch?v=dPzXvr6gULc

OP would need to use a compass to scribe because of the popcorn ceiling rather than the scribe that Paul is using in the video.

Like a previous user said, paintable caulk is a huge benefit here- Big Stretch is great for this purpose. Would never recommend anything fast-drying, as it will crack. Make sure to wait before painting for the sealant to fully cure. Another amazing option is eXact color tintable caulk which is a caulk that you can add paint to to match your paint color exactly- no painting over required.