If it makes any difference, I'm going to need to do some evening on the
panels I glued together (there is a little variation here and there that I
need to make flat) - either with heaving sanding or trying to hand plane to
flat.

I would pass them through a planer if available until all are of the
same thickness - regardless of whether they need it or not.

That is, all shelves should be equal in thickness to look ok.

You mean only screws will be holding up the shelves? If so, use at
least 4 screws per side if they are to hold books. Next time, let them
in (dado).

Lou